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Compound  Formation,  Solubility,  and 
lonization  in  Fused  Salt  Mixtures 


1.     Compound  Formation  between  Aluminium  Bromide 
and  Other  Bromides. 


By 
EUGENE  D.  CRITTENDEN,  B.A.,  M.A. 


DISSERTATION 

Submitted  in  Partial  Fulfillment  of  the  Requirements  for  the 

Degree  of  Doctor  of  Philosophy  in  the  Faculty  of 

Pure  Science,  Columbia  University  in  the 

City  of  New  York 


NEW   YORK    CITY 
1922 


Compound  Formation,  Solubility,  and 
lonization  in  Fused  Salt  Mixtures 


1.    Compound  Formation  between  Aluminium  Bromide 
and  Other  Bromides. 


By 
EUGENE  D.  CRITTENDEN,  B.A.,  M.A. 


DISSERTATION 

Submitted  in  Partial  Fulfillment  of  the  Requirements  for  the 

Degree  of  Doctor  of  Philosophy  in  the  Faculty  of 

Pure  Science,  Columbia  University  in  the 

City  of  New  York 


NEW   YORK   CITY 
1922 


ACKNOWLEDGMENT 

The  author  wishes  to  express  to  Professor  James  Kendall,  at 
whose  suggestion  this  investigation  was  undertaken,  his  sincere 
gratitude  for  his  constant  advice  and  help  throughout  the  course  of 
the  work. 

The  author  also  desires  to  thank  the  other  members  of  the 
Chemistry  Department  of  Columbia  University  for  their  interest 
and  cooperation. 


ABSTRACT  OF  DISSERTATION 

1.  What  was  attempted? 

2.  In  how  far  were  the  attempts  successful? 

3.  What  contributions  actually  new  to  the  science  of  chemistry  have 
been  made? 

1.  The  attempt  was  made  to  demonstrate  the  application  of 
rules  already  laid  down  regarding  compound  formation  and  solubility 
to  the  case  of  fused  salt  mixtures.     Solubility  data  for  twenty-five 
systems  in  which  aluminium  bromide  was  used  as  a  solvent  for 
other  bromides  have  been  presented. 

2.  It  has  been  shown  that  compound  formation  and  solubility 
for  salt  mixtures  varies  in  accordance  with  the  "diversity"  of  the 
positive  radicals,  reaching  a  minimum  in  the  vicinity  of  aluminium 
and  increasing  markedly  in  either  direction  from  this. 

3.  (a)  The  influence  of  subsidiary  factors,  such  as  unsatura- 
tion,  valence,  and  temperature  of  fusion  on  the  formation  of  addition 
compounds  has  been  outlined. 

(b)  The   effect   of   internal   pressure   and   atomic   volume   on 
miscibility  and  compound  formation  has  been  fully  discussed. 

(c)  In  the  course  of   the  investigation  thirty-two  new  com- 
pounds have  been  isolated. 


COMPOUND  FORMATION,  SOLUBILITY,  AND  IONIZA- 
TION  IN  FUSED  SALT  MIXTURES 

1.  Compound  Formation  between  Aluminium  Bromide  and  Other 
Bromides. 

In  a  recent  series  of  papers  by  Kendall  and  his  coworkers1 
extensive  evidence  has  been  presented  correlating  compound  forma- 
tion, solubility,  and  ionization  in  solution.  It  has  been  postulated 
that  compound  formation  is  most  marked  and  consequently  solubility 
is  most  extensive  in  systems  where  the  radicals  are  most  diverse 
in  character;  the  criterion  of  diversity  being  their  position  in  the 
electrode  potential  series.  Thus  Landon2  and  Davidson3  found 
in  the  systems  sulfuric  acid-metal  sulfate  that  potassium  and  silver, 
the  metals  farthest  removed  from  hydrogen  in  either  direction,  gave 
the  greatest  compound  formation.  This  "diversity"  rule  has  been 
tested  for  a  large  number  of  systems  and  has  been  found  to  hold 
without  marked  exception.  A  few  minor  discrepancies,  however, 
have  come  to  light;  and  in  order  to  explain  these,  several  additional 
factors,  such  as  atomic  volume,  internal  pressure,  and  unsaturation, 
which  will  be  taken  up  more  fully  below,  have  been  mentioned. 

In  order  to  obtain  additional  evidence  regarding  the  rules 
already  laid  down,  it  was  thought  best  to  study  next  systems  where 
double  salts  should  be  formed.  For  this  purpose,  as  reference, 
a  salt  of  low  melting  point  was  desirable  as  well  as  one  whose 
metallic  constituent  was  above  hydrogen  in  the  "electrode  potential" 
series  and  whose  radicals  were  as  diverse  as  possible.  Aluminium 
bromide  seemed  to  give  promise  of  fulfilling  such  requirements,  and 
twenty-five  systems  using  this  salt  as  a  solvent  have  been  investigated 
by  the  freezing  point  method,  using  sealed  bulbs  throughout.  It 
was  again  deemed  advisable  to  follow  the  order  of  elements  as 
given  by  the  "electrode  potential"  series,  although  this  is  known 
to  be  in  error  in  several  instances.4 

The  "diversity"  factor  has  been  found  to  hold  as  well  for 
these  systems  as  for  those  previously  reported.  This  "diversity" 
factor,  although  the  main  point  upon  which  to  base  any  comparison 
of  results  in  systems  where  compound  formation  occurs,  must  be 
joined  up  with  several  subsidiary  factors  such  as  internal  pressure, 
atomic  volume,  unsaturation  of  the  radicals,  and  the  temperature  of 
fusion  of  the  pure  substances  in  order  to  obtain  a  satisfactory  correla- 

1  See  Kendall  and  Davidson   (A.W.),  J.A.C.S.,  43  980  (1921)    for  refer- 
ences  to  earlier   work. 

2  Landon,    Columbia   University   Dissertation,    1920. 

3  Davidson,    Columbia   University  Dissertation,    1920. 
'Kendall,  Davidson,  and  Adler,  J.A.C.S.,  43  1501   (1921). 


tion  of  this  work  with  the  results  previously  given.     It  is  with  this 
object  in  view  that  the  present  work  is  presented. 

Previous  Work  on  Fused  Salts 

The  literature  is  crowded  with  examples  of  double  salts,  but 
far  too  often  the  work  was  carelessly  done  or  carried  out  in  the 
presence  of  solvents,  no  account  of  whose  action  was  taken.  No 
systematic  investigation  on  binary  systems  of  fused  salts  has  ever 
been  performed  with  the  idea  of  correlating  fact  with  theory. 
Many  isolated  systems  in  aqueous  solution  are  recorded,  but  most 
often  the  compounds  reported  were  hydrates  of  uncertain  compo- 
sition and  stability  or  else  haphazard  mixtures  were  made  up  and 
each  result  called  a  compound.1  Obviously,  such  results  can  not 
be  used  in  a  rigorous  study  of  solubility  rules.  That  double  salts 
are  extensively  formed  in  solutions  is  evidenced  by  the  large  number 
of  different  types  of  such  salts  reported.  Such  cases  as  the  double 
cyanides  with  potassium  cynide,  the  alums,  and  the  platinocyanides 
furnish  only  a  few  examples  of  such  types.  It  is  interesting  to  note 
that  in  the  above-mentioned  cases  the  greatest  number  and  the  most 
stable  compounds  are  formed  with  those  salts  whose  metallic  con- 
stituents are  most  diverse  from  the  reference  metal,  hence  presenting 
further  evidence  that  the  diversity  factor  is  the  main  one  in  any 
consideration  of  systems  where  compound  formation  occurs.  Thus 
we  find  potassium  argentocyanide,  potassium  and  barium  platino- 
cyanides, and  the  alums,  with  the  alkali  metals,  are  the  most  stable 
compounds  in  their  respective  systems. 

Numerous  double  bromides  are  also  recorded,  but  here  again 
the  method  of  their  isolation  and  study  makes  their  composition  so 
uncertain  that  they  are  not  included  in  a  discussion  here.  Miller2 
has  correlated  the  data  for  chloride  systems  and  it  is  apparent  that 
the  bromides  do  not  present  a  far  different  case.  It  was  hoped 
that  the  present  work  would  lead  towards  the  establishment  of  the 
position  of  fused  salts  in  the  general  scheme  of  compound  forma- 
tion, solubility,  and  ionization  already  presented.3  Such  hopes  have 
been  realized  for  the  cases  of  compound  formation  and  solubility 
when  account  is  taken  of  the  subsidiary  factors;  i.e.,  atomic  volume, 
internal  pressure,  valence,  etc.,  which,  together  with  the  "diversity" 
factor  already  fully  discussed,4  constitute  the  basis  for  such  com- 
parisons. 

Choice  df  Aluminium  Bromide  as  a  Solvent 

Most  salts  present  so  many  shortcomings  as  solvents  that  the 
choice  of  one  suitable  to  fulfil  a  given  set  of  conditions  is  by  no 

1  An  interesting  case  is   found  in  the  work  of  Baud,  Ann.   Chim.   Phys., 
(8)    /  8.    (1904). 

2  Miller,  Columbia  University  Dissertation   (1922). 

'Kendall  and  Gross,  J.A.C.S.,  43  1416  (1921).    Also  Kendall  and  David- 
son,  T.A.C.S.,  43  980   (1921). 

'"Kendall,  Booge,  and  Andrews,  J.A.C.S.,  39  2303  (1917). 

8 


means  easy.  In  order  to  make  the  present  investigation  as  thorough 
as  possible,  the  careful  selection  of  a  salt  with  just  the  proper  set 
of  characteristics  was  necessary.  This  salt  must  have  a  low  internal 
pressure,  be  non-polar  in  character,  have  a  low  melting  point,  and 
possess  radicals  which  are  as  widely  diverse  as  practicable.  A  salt 
of  a  higher  valence  type  than  uni-univalent  was  also  desirable,  as 
well  as  one  whose  metallic  constituent  had  a  position  well  above 
hydrogen  in  the  "electrode  potential"  series. 

After  an  extended  search,  aluminium  bromide  was  finally 
selected  as  most  nearly  fulfilling  the  above  requirements.  It  has 
a  low  melting  point,  a  low  internal  pressure  as  evidenced  by  its 
position  in  a  table  of  relative  internal  pressures,1  is  slightly  polar, 
and  satisfies  the  requirements  of  diversity  and  position  of  its 
radicals.  Kablukow2  and  Menschutkin3  have  already  shown  that 
aluminium  bromide  is  an  excellent  solvent  for  a  large  number  of 
organic  substances.  They  have  isolated  several  compounds,  but 
only  in  those  systems  where  such  a  result  would  be  expected  from 
the  rules  already  laid  down.  Isbekow  and  Plotnikow4  in  a  paper 
entitled  "Aluminium  Bromide  as  a  Solvent"  have  presented  con- 
siderable evidence  of  a  qualitative  nature  in  regard  to  the  solvent 
action  of  aluminium  bromide  for  inorganic  salts.  These  writers 
did  not  make  a  complete  study  of  any  of  the  systems  presented, 
but  were  only  interested  in  relative  solubilities  with  the  idea  of  using 
such  information  later  as  a  basis  for  conductivity  determinations. 
Conductivity  results  have  been  given  for  three  systems,  but  the 
discussion  of  their  bearing  on  this  work  will  be  left  till  after  a  dis- 
cussion of  the  present  results. 

Experimental  Procedure 

In  the  present  investigation  solutions  of  the  bromides  of  Li, 
Na,  K,  NH4,  Ag,  Ca,  Ba,  Mg,  Zn,  Cd,  Hg',  Hg",  Tl',  C,  Sn"  ", 
Sn",  Pb",  As"  ',  Sb"  ',  Bi,  P"  ',  Cr"  ',  Mn",  Fe",  and  Ni  in  alumin- 
ium bromide  have  been  studied  by  the  freezing  point  method.5 
The  solubilities  in  all  but  a  few  cases ;  i.e.,  Ni  and  Cr"  ',  were  quite 
considerable  and  the  curves  have  all  been  determined  at  least  to 
20%,  with  the  exception  of  the  two  above.  With  the  alkali  metals, 
for  example,  the  curves  have  been  extended  beyond  50%,  after 
which  the  slope  of  the  curve  becomes  very  steep;  a  point  which 
will  be  discussed  under  the  lithium  system.  In  other  cases  the 
curves  were  carried  at  least  far  enough  to  give  evidence  of  a 
compound  such  as  2AlBr3,  MBrx,  if  such  existed.  With  bromides 
of  low  melting  point  the  complete  curves  were  determined. 

1  See  page  48  for  table  of  internal  pressures  of  bromides. 

2  Kablukow  and  Khanow,   Chem.  Zentral   Blatt,    (1)    419-32,    (1910). 

3  Menschutkin,  J.  Chim.  Phys.,  10  552  (1911). 

4  Isbekow  and  Plotnikow,  Zeit,  Anorg.  Chem.,  71  328   (1911). 
"Kendall  and  Booge,  J.A.C.S.,  38  1718  (1916).    Also  Kendall  and  Landon, 

J.A.C.S.,  42  2131   (1920). 


Due  to  the  exceedingly  hygroscopic  nature  of  aluminium 
bromide,  sealed  bulbs  were  used  throughout  and  the  preparation 
of  such  tubes  confined  to  clear,  cold,  dry  days.  For  this  purpose  it 
was  thought  advisable  to  depart  from  the  method  of  sealing  on 
a  handle  after  preparing  and  filling  the  bulb,  as  had  been  done  by 
previous  workers.1  The  method  adopted  was  to  blow  a  bulb  on 
the  end  of  a  Pyrex  tube  from  12-15  cms.  in  length,  depending  on 
the  temperature  at  which  the  tube  was  to  be  used.2  The  diameter 
of  the  tube  also  varied  from  3-6  mms.,  depending  on  the  type  of 
substance — liquid  or  solid — to  be  used.  The  tubes  were  all  care- 
fully cleaned  and  dried  before  filling.  The  tubes  were  first 
weighed  empty  and  then  the  substance  to  be  determined  was  added. 
After  reweighing,  the  aluminium  bromide,  which  was  kept  in  a 
weighing  bottle  over  phosphorous  pentoxide,  was  quickly  added  and 
the  tube  sealed  in  a  blast  lamp.  After  cooling,  the  tubes  were 
washed,  dried,  and  reweighed.  From  the  weights  so  obtained,  the 
percentage  composition  could  be  readily  computed.  After  short 
experience  little  trouble  was  found  in  judging  the  proper  quantities 
of  the  two  components  needed  to  make  up  a  desired  composi- 
tion. Great  care  was  exercised  to  insure  no  adherence  of  the  sub- 
stance to  the  upper  part  of  the  tube,  since  this  would  introduce 
a  serious  error.  With  a  very  fine  powder,  such  as  mercurous 
bromide,  it  was  found  necessary  to  use  a  clean  funnel  brush  to 
remove  adhering  powder  from  the  upper  part  of  the  tube.  Usually, 
however,  light  tapping  was  found  sufficient,  since  most  substances 
were  used  as  crystals  not  too  finely  powdered. 

The  tube,  after  the  final  weighing,  was  immersed  in  a  bath 
whose  temperature  was  carried  several  degrees  beyond  the  disap- 
pearance of  the  last  trace  of  solid.  Thorough  agitation  was  main- 
tained by  tilting  and  shaking.  Each  point  was  determined  at  least 
twice,  the  disappearance  of  the  last  trace  of  crystals  being  taken 
as  the  melting  point  for  that  composition. 

The  bath  in  which  the  bulb  was  placed  for  the  determination 
of  the  melting  point  varied  with  the  temperature  range  in  which 
the  point  lay.  The  baths  used  with  their  respective  temperature 
ranges  were: 

HNO3  and  Ice —25°—    0° 

Water   0°—  30° 

Sulfuric  Acid   30°— 110° 

30%  (NH4)2SO4  and  70%  H2SO4....     110°— 200° 

NaNO3,  KNOS,  and  Ca(NO3)2 200°— 310° 

3NaNO,  and  KNCX.  310°— 550° 


1  Davidson,  Columbia  University  Dissertation,    (1920). 

2  Longer  tubes  were  found  necessary  for  temperatures  above  180°. 

8  See  Menzies  and  Nutt,  J.A.C.S.,  33  1366   (1911),  for  eutectic  mixtures 
of  nitrates. 

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In  work  of  such  wide  range  considerable  attention  had  to  be 
paid  to  the  factors  affecting  thermal  equilibrium  between  the  tube, 
thermometer,  and  the  bath.  It  was  found  necessary  to  observe  the 
following  precautions  in  order  to  avoid  errors  which  would  ap- 
preciably affect  the  freezing  points. 

1.  The    tube    and    bath    were    stirred    constantly    during    the 
determination  of  a  point. 

2.  The  temperature  was  changed  slowly  enough  to  maintain1 
as  nearly  as  practicable  thermal  equilibrium  during  the  heat- 
ing process. 

3.  The  bulbs  were  sufficiently  thin  to  prevent  lag  when  the 
above  precautions  were  observed.2 

4.  The  air  space  in  the  bulbs  was  as  small  as  possible  to  avoid 
excess  pressure. 

At  higher  temperatures  draughts  and  radiation  were  excluded 
by  the  use  of  an  asbestos  shield  surrounding  the  bath.  Windows 
were  provided  in  this  to  permit  observation. 

Measurement  of  Temperature 

Temperatures  were  measured  by  means  of  two  mercury  ther- 
mometers graduated  in  fifths  of  degrees,  having  ranges  0° — 200° 
and  200° — 300°  respectively.  For  temperatures  above  300°  a  base 
metal  pyrometer,  with  a  low  resistance,  direct  reading  galvanometer 
was  used  which  had  a  scale  graduated  in  10°  intervals.  The 
accuracy  and  corrections  for  the  thermometers  and  pyrometer  were 
determined  by  the  use  of  a  platinum  resistance  thermometer3  care- 
fully calibrated  against  the  freezing  and  boiling  points  of  water  and 
the  boiling  point  of  sulfur  by  the  method  prescribed  by  the  Bureau 
of  Standards.4  The  value  of  "delta"  as  found  was  1.63,  which 
is  sufficiently  low  to  permit  the  use  of  the  thermometer  as  a  stand- 
ard.5 The  standard  temperature-resistance  curve  was  plotted  by 
means  of  the  data  obtained  and  the  temperature  for  any  resistance 
could  then  be  read  directly  from  this.  Intermediate  points  were 

R 
determined  by  means  of  the  formula:  —  =  1  +  at  +  bt,2  where 

RO 

R0  is  the  resistance  at  0°,  R  is  the  resistance  at  the  temperature  t, 


1  About   0.2°    per  minute  was    the  average   rate   of    heating.     This   was 
subject  to  slight  variation  depending  on  the  slope  of  the  curve.     Thus  for  a 
steep  slope  the  rate  of  heating  was  slightly  increased,  while  for  the  reverse 
case  it  was  diminished. 

2  This  is  shown  by  the  smoothness  of  the  curve  for  any  given  phase. 

8  For  the  use  of  the  thermometer  and  accessories,  the  author  wishes  to 
express  his  thanks  to  Prof   C.   D.   Carpenter. 

4  For  the  method  of  assembling  and  use,  see  Reprint  No.  124,  Bureau  of 
Standards.     Also  J.A.C.S.,  41  748.     (1919). 

5  The  standard  value  for  pure  platinum  is  1.50,  but  a  slightly  higher  value 
does  not  alter  the  constants  a  and  b  sufficiently  to  affect  this  work  or  any 
other  except  the  most  precise. 

12 


and  a  and  b  are  constants  determined  from  the  standard  data  for  the 
thermometer. 

The  thermometers  were  immersed  to  the  same  depth  during  the 
calibration  as  during  their  use  throughout  this  investigation.  By 
determining  the  resistance  at  10°  intervals,  the  necessary  correction 
for  any  temperature  could  be  easily  found  by  taking  the  difference 
between  this  resistance  and  that  obtained  from  the  standard  calibra- 
tion curve.  Thus  both  stem  correction  and  standardization  for 
the  thermometers  were  directly  determined  at  the  same  time.  By 
plotting  these  corrections  against  the  temperatures,  the  necessary 
correction  for  any  point  on  the  thermometers  could  be  read  directly. 
In  the  case  of  the  pyrometer,  the  calibration  was  made  for  50°  inter- 
vals since  relative  values  only  were  required  above  300°. 

PRECISION  OF  MEASUREMENTS 

The  freezing  points  of  mixtures  prepared  by  the  method  de- 
scribed above  and  determined  by  means  of  one  of  the  thermometers 
or  the  pyrometer  have  definite  precision  values.  These  values  depend 
on  the  temperature  range  in  which  the  point  lies  and  also  on  the 
slope  of  the  curve.  The  approximate  limits  of  accuracy  obtained 
in  this  work  are  as  follows : 

Sealed  bulb 0°— 100°  0.3°— 0.5° 

Sealed  bulb 100°— 200°  0.5°— 1°.0 

Sealed  bulb 200°— 300°  1°.0— 2°.0 

Sealed  bulb above      300°  2°.0— 5°.0 

The  higher  limits  above  refer  to  the  accuracy  of  the  tempera- 
ture measurements,  taking  into  account  all  probable  sources  of  error ; 
i.e.,  constant  and  personal  errors,  as  well  as  errors  of  method.  The 
lower  limit  expresses  the  reproducibility  of  the  values. 

The  composition  data  have  an  accuracy  of  one-tenth  of  a  molec- 
ular per  cent,  the  hygroscopic  nature  of  most  of  the  salts  employed 
making  it  impossible  to  obtain  better  values. 

EXPERIMENTAL  RESULTS 
Preparation  of  Aluminium  Bromide. 

Pure  aluminium  bromide  was  prepared  by  dropping  30  mesh 
aluminium  slowly  into  bromine  to  avoid  too  vigorous  action.  When 
the  action  was  complete,  excess  aluminium  was  added  and  the 
product  digested  for  an  hour  over  a  low  flame.  The  salt  was  dis- 
tilled twice  from  excess  aluminium  and  then  transferred  by  distilla- 
tion into  a  large  flask  with  side  arm  and  ground  glass  stopper. 
Throughout  this  work  a  calcium  chloride  tube  was  connected  to  the 
side  arm  of  the  flask. 

In  this  flask  the  aluminium  bromide  was  sublimed  on  a  water 
bath  containing  salt  solution  which  boiled  above  105°.  When  suffi- 
cient product  had  collected,  it  was  shaken  loose  and  transferred  to  a 

13 


clean,  dry  weighing  bottle,  which  was  kept  over  phosphoric  anhydride 
in  a  vacuum  desiccator  until  used.  Only  small  quantities  were  pre- 
pared at  one  time  due  to  the  rapid  decomposition  of  the  salt  on 
exposure  to  the  air.  The  salt  becomes  light  yellow  after  it  has  taken 
up  even  a  trace  of  water,  and  this  was  used  as  one  criterion  of 
purity,  no  sample  being  used  after  the  first  trace  of  yellow  appeared. 
The  salt  was  obtained  in  white  shining  plates  which  melted  uni- 
formly at  97°.  1.  This  value  is  considerably  higher  than  those  previ- 
ously recorded.1  Although  not  ultra  pure,2  the  salt  was  sufficiently 
pure  for  freezing  point  work.  Throughout  this  investigation  the 
melting  point  of  the  aluminium  bromide  never  fell  below  96°. 8,  as  was 
shown  by  tests  made  at  frequent  intervals.  By  use  of  the  thermal 
method,  a  new  form  of  aluminium  bromide  with  a  transition  point 
at  70°. 2  was  found.  This  new  modification  has  been  confirmed  in 
several  systems  below. 

SYSTEM  LiBr— AlBr3 

Pure  lithium  bromide  was  prepared  from  a  U.S. P.  sample  by 
precipitating  the  carbonate  and  redissolving  this  in  just  sufficient  c.p. 
HBr  to  affect  solution.  The  solution  was  evaporated  to  small  volume 
and  allowed  to  crystallize.  The  mother  liquor  was  removed  as  com- 
pletely as  possible  by  decantation  and  the  crystals  of  the  hydrate 
transferred  to  a  Pyrex  tube — 1  cm.  by  10  cm.  The  tube  was  heated 
slowly  at  first  to  decompose  the  hydrates  and  then  rapidly  to  a  tem- 
perature above  600°.  After  cooling,  the  tube  was  carefully  broken 
and  only  the  large  lumps  taken  in  order  to  avoid  contamination  by 
glass.  The  salt  was  kept  in  a  glass  stoppered  bottle  in  an  oven  at 
120°  during  the  whole  course  of  the  work  on  this  system.3  Due  to 
the  exceedingly  hygroscopic  nature  of  lithium  bromide,  tubes  for  this 
system  were  prepared  only  on  'exceptionally'  clear,  dry  days. 

No  previous  work  on  this  system  could  be  found  in  the  literature. 

The  data  for  this  system  are  given  below  and  in  Fig.  1,  B. 

(a)  Solid  phase — AlBr3. 
%LiBr.  0.00 
4T.                     97.1 

(b)  Solid  phase— 7AlBr,,LiBr. 

%LiBr.  .62        2.0         4.3          8.7        11.1        14.0        16.2 

T.  107.2      108.2      109.8      112.4      113.6      114.6      113.0 

1  Varying  values  are  given  by  different  investigators.     Thus  90°,  Weber, 
Pogg.  Ann.  w 3,  204.   (1857)  ;  93°.0,  Abegg,  Handbuch  Anorg.  Chem.,  3   (1) 
75;  96°.0,  Menschutkin,  loc.  cit. 

2  Richards   and   Krepelka,  J.A.C.S.,  43  2221    (1920)    have  used  this   salt 
for  atomic  weight  determinations,  but  they  report  no  value  for  the  melting 
point. 

*  This  general  policy  was  adopted  for  all  systems  where  the  salts  were 
very  hygroscopic  and  possessed  a  sufficiently  high  fusion  point,  (e.g.  above 
150°). 

4  T.  is  the  temperature  of  the  disappearance  of  the  last  trace  of  crystals ; 
taken  as  the  fusion  point  for  that  composition. 

14 


f/G.M 


0 

>0 

w 

'0 

w 
w 
to 

'0 

10 
0 
0 
'0 

0 
C 

A 

/ 

x-*  D 

• 

/ 

f* 

^Br- 
^Pb  Br2 
'SnBr2 
[Ca  Br2 

-AlBr^ 

Tnl  BPj, 

Add< 
!5ubtra 

>0'to7 
O.K. 
rflSo'fi 
270* 

r— 

V 

1 

B( 
C( 

emp. 
vnTemp. 

O( 

x^ 

». 

c 

x^ 

.*-• 

y 

-N^" 

•\ 

s./: 

X 

/ 

'""N 

V                         / 

/ 

—  x 

/ 

\/ 

Jx< 

s 

c 

\ 

A 

7 

I 

\ 

/ 

! 

l                    20                  40                   60                   80                  /a 

1  Br3                                                                                                              Added  Brwnid* 

MOLECULAR 


15 


(c)  Solid  phase— 2AlBrs,LiBr. 

%LiBr.              14.0        17.1        22.0  25.7  27.8  28.7        30.6 

T.               114.6      117.7      121.9  125.2  126.7  127.9      129.4 

(d)  Solid  phase— AlBr3,LiBr. 

%LiBr.              34.2        37.9        39.1  40.7  44.9  46.8        48.4        50.6 

T.               135.5      152.2      157.9  164.5  180.7  186.7      192.5      195.4 

(e)  Solid  phase— Li Br. 

%LiBr.              51.4        56.7        63.9  74.6  90.0  100.0 

T.               221.4      405.0      510.0  523.0  532.0  535.0 

The  composition  in  this  and  following  systems  is  expressed 
throughout  in  molecular  percentages.  It  was  deemed  advisable  to 
carry  the  work  far  enough  to  complete  the  curve  since  it  is  rising 
sharply  beyond  the  50%  compound.  As  had  been  expected,  no  com- 
pound with  more  than  50%  LiBr  was  found  in  this  system,  a  point 
to  be  discussed  later.  The  curve  rises  rapidly  till  70%  LiBr  is 
reached,  after  which  the  rate  of  change  is  slow  up  to  the  melting 
point  of  pure  lithium  bromide  at  535°. 

Three  new  compounds,  7  AlBr3,  LiBr ;  2AlBr3,  LiBr ;  and  AlBr3, 
LiBr,  have  been  isolated  in  this  system.  The  first  is  stable  just  up 
to  its  maximum  at  114°. 4,  the  second  undergoes  transition  to  the 
equi-molecular  compound  before  its  maximum  is  reached,  while  the 
third  is  a  stable  compound  with  a  M.P.  197°. 

It  will  be  noted  from  the  figure  (Fig.  1,  B)  that  the  curve  rises 
sharply  at  first  with  small  changes  of  composition,  but  soon  flattens 
out.  This  behavior  is  typical  for  cases  where  two  liquid  layers  are 
present  in  the  metastable  region.1 

SYSTEM  NaBr-AlBr3 

The  salt  used  was  a  c.p.  sample  recrystallized  once  from  dis- 
tilled water  and  carefully  dried  in  an  oven  at  120°. 

Isbekow  and  Plotnikow2  have  found  that  NaBr  is  quite  soluble 
in  aluminium  bromide  even  at  100°.  They  report  the  breaking  up 
of  mixtures  with  small  percentages  into  two  liquid  layers  with  a 
critical  temperature  of  mixing  near  230°.  They,  however,  report 
no  compounds. 

The  present  work  confirms  the  existence  of  a  two  liquid  layer 
region  with  low  percentages  of  NaBr.  The  solid  phase  separating 
out  from  this  region  has  been  shown  to  contain  an  unstable  com- 
pound3 as  evidenced  by  the  double  eutectic. 

The  data  for  this  system  are  given  below  and  in  Fig.  1,  E. 

(a)  Solid  phase— AlBr3. 

%NaBr.  0.0         0.7         0.9          1.4          1.7 

T.  97.1        95.2       94.8       93.6       93.0 


1  See  Miller,  loc.  cit.,  for  an  interesting  case  of  this  kind. 

2  Isbekow  and  Plotnikow,  loc.  cit. 

8  For  a  discussion  of  unstable  compounds  of  this  very  rare  type,  see  Rooze- 
boom,  Heterogene  Gleichgewicht,  2   (2)    168. 

16 


(b)  Solid  phase— xA!Br3,yNaBr. 
%NaBr.  1.9          2.2 

T.  93.9        94.6 

(c)  Two  liquid  layers;  solid  phase  disappearing  at  95°. 4. 

%NaBr.  3.1          4.0          5.4         7.8        10.45      12.8        14.5        15.3 

lT.ofCoales   110.9      166.4      227.1      231.9      230.5      202.7      166.9      125.6 

(d)  Solid   phase— xA!Br3,y  NaBr2. 
%NaBr.  17.0        17.7        18.4        19.0 

T.  94.2       93.0        91.6        90.4 

(e)  Solid  phase— 7AlBr3,2NaBr. 

%NaBr.  16.9        18.4        20.5        21.5        22.1.       23.4 

T.  92.8       94.0        95.4       95.8        96.0        95.1 

(f)  Solid  phase— 2AlBr,,NaBr. 

%NaBr.  24.03      25.1        27.5        31.9 

T.  94.8        98.2      102.4      104.5 

(g)  Solid  phase— AlBr3,NaBr. 

%NaBr.  32.6        35.3        36.8        38.9        42.0        45.0        48.6        50.3 

T.  108.8      131.2      141.5      154.8      170.4      184.0      196.4      200.5 

(h)   Solid  phase— NaBr. 

%NaBr.  50.3        51.1 

T.  269.0      360.0 

As  will  be  seen  from  the  diagram  (Fig.  1,  E),  the  curve  is 
rising  rapidly  beyond  the  50%  composition  as  was  the  case  with  the 
lithium  system  above.  This  precludes  the  possibility  of  a  higher 
compound  than  an  equimolecular  since  the  solid  phase  separating  is 
unquestionably  NaBr.3 

Four  new  compounds  not  previously  reported  have  been  found 
in  this  system.  The  unstable  compound  of  uncertain  composition 
in  the  two  layer  region  has  already  been  mentioned.  The  com- 
pound 7AlBr3,  2NaBr  is  stable  at  its  maximum  and  has  a  melting 
point  of  95°. 6,  while  the  compound  2AlBr3,  NaBr  undergoes  transi- 
tion to  the  equimolecular  before  its  maximum  is  reached.  The 
latter  is  just  stable  at  its  maximum  and  has  a  melting  point  of  201°. 

SYSTEM  KBr-AlBr3 

The  salt  used  was  a  c.p.  sample  recrystallized  once  from  distilled 
water  and  carefully  dried. 

Isbekow  and  Plotnikow4  have  found  that  this  salt  is  also  quite 
soluble  in  aluminium  bromide  at  100°,  giving  two  liquid  layers  in 
the  low  percentages.  They  were  unable  to  determine  the  critical 
temperature  of  mixing  on  account  of  its  high  value  (above  360°). 
They  report  no  compounds  in  their  work.  Weber5  found  an  equi- 

1  Temperature   of   coalescence   is   that   temperature   at   which  one   of   the 
two  liquid  layers  disappears. 

Throughout  this  work  the  above  type  of  formula  will  be  used  to  desig- 
nate the  compound  of  uncertain  composition  in  the  two  layer  region. 

3  The  behavior  of  this  system  beyond  the  50%  composition  is  similar  to 
that  of  lithium  in  the  same  region  where  it  has  been  shown  that  no  compounds 
exist. 

4  Isbekow  and  Plotnikow,  loc.  cit. 
5  Weber,  Pogg.  Ann.  103  259. 

17 


molecular  compound  by  fusing  together  potassium  and  aluminium 
bromides,  distilling  off  the  excess  aluminium  salt  and  analyzing  the 
residue. 

This  investigation  confirms  the  work  of  both  Isbekow  and 
Weber  as  well  as  adding  two  new  compounds,  an  unstable  one  in 
the  two  layer  region  and  2AlBr3,  KBr,  both  not  previously  reported. 

The  data  for  this  system  are  given  below  and  in  Fig.  1,  C. 

(a)  Solid  phase— AlBr,. 
%KBr.  0.00 

T.  97.1 

(b)  Solid  phase— xAlBr»yKBr. 
%KBr.  0.33 

T.  97.5 

(c)  Two  liquid  layers;  solid  phase  disappearing  at  98°.l 
<&KBr.  0.86        19.4 

T.  of  C.         265.9      189.6 

(d)  Solid  phase— xA!Br3,yKBr. 
%KBr.  23.3        24.7 

T.  95.6       92.2 

(e)  Solid  phase— 2AlBr3,KBr. 

%KBr.  25.8        27.1        28.8        31.6        33.2 

T.  88.6       90.8       93.0       95.5        95.8 

(f)  Solid  phase— AlBr8,KBr. 

%KBr.  34.7        37.3        39.5        41.0        44.5        47.4        49.0      51.26 

T.  109.0      130.6      145.9      154.6      171.4      181.7      188.4    189.6 

(g)  Solid  phase— KBr. 
%KBr.  52.0  54.7 

T.  188.8  above  390. 

Here  again  the  curve  rises  rapidly  beyond  the  50%  composition, 
KBr  separating  as  the  solid  phase.  With  the  exception  of  the  equi- 
molecular  compound,  which  gets  slightly  beyond  the  50%  composi- 
tion, the  compounds  are  not  stable  at  their  maxima.  The  compound 
2AlBr3,  KBr  undergoes  transition  to  the  equimolecular  compound 
at  96°  and  33%.  The  equimolecular  compound  melts  at  191°.5. 

SYSTEM  NH4Br-AlBr3 

This  salt  was  prepared  from  a  c.p.  sample  by  one  recrystalliza- 
tion  from  distilled  water  and  subsequent  thorough  drying. 

Isbekow  and  Plotnikow1  found  exactly  the  same  behavior  for  this 
salt  in  aluminium  bromide  as  for  potassium ;  a  result  to  be  expected 
from  their  similar  action  previously  noted.2  It  was  impossible  to 
cause  the  coalescence  of  a  mixture  containing  only  5%  NH4Br  even 
at  the  boiling  point  of  aluminium  bromide  (365°).  The  critical 
temperature  of  mixing  for  this  system  must  lie  considerably  higher 
than  this,  thus  confirming  the  observation  of  Isbekow.  Four  new 
compounds  not  previously  recorded  have  been  isolated.  As  with 

1  Isbekow  and  Plotnikow,  loc.  cit. 

2  See  Kendall  and  Landon,  loc.  cit.,  for  a  comparison  of  the  corresponding 
sulfate  systems.    Also  Kendall  and  Adler,  loc.  cit.,  for  the  formate  systems. 

18 


F/G.JOT 


BMP 

550 
500 
450 

y  400 
UJ 

^350 

1 
g~ 

250 
200 
150 
100 
50 

°< 

/ 

1 

c 

Q 

\ 

~~*.. 

/ 
• 

/, 

\ 

7 

r 

'HgBr2- 
'CdBr2- 
'MnBr,- 
'HgBr- 

AlBrjl 
Al  Br3) 
AlBrj) 
Al  Br3) 

Add? 
«   5 

Subfrac 

M 

O'toT* 
0°- 
*IOO*f™ 
250°- 

A( 
B( 
C( 

?mp. 

9* 

mTemp. 

r       *• 

^ 

X" 

D( 

r 

1 

1 

E 

^_^,» 

l^ 

\ 

9* 

^^•"^"^ 

r 

\ 

X 

^^•"  — 

^^ 

/ 

X 

\ 

A 

j.  .— 

—  ••—  «^- 

y 

•*^_-  •—  • 

20                         4V                         60                         60                         101 
V  8r3                                                                                               Added  Bromide 

MOLECOLAR  PERCENTAGE 

19 


potassium,  an  unstable  compound  exists  in  the  two  layer  region,  show- 
ing this  to  be  the  rule  for  the  alkali  metals  investigated.1 

The  data  for  this  system  are  given  below  and  in  Fig.  1,  D. 

(a)  Solid  phase — AlBr3. 
%NH4Br.  0.00 

T.  97.1 

(b)  Two  liquid  layers;  solid  phase  disappears  at  98°. 0. 
%NH4Br.  0.52        0.8        20.45 

T.ofCoales.    160.5      236.5      159.5 

(c)  Solid  phase— xAlBr3,yNH4Br. 
%NH4Br.          24.0 

T.  94.8 

(d)  Solid  phase— 3AlBr3,NH4Br. 

%NH4Br  22.8        23.1        23.8        25.2        27.3 

T.  96.6        96.9        97.5        97.3        92.7 

(e)  Solid  phase— 2AlBr3,NH4Br. 

%NH4Br.          27.3        28.2        31.0        32.4        32.9        33.5 
T.  98.1        99.9      103.2      103.8      104.0      103.6 

(f)  Solid  phase— AlBr3,NH4Br. 

%NH4Br.          36.3        38.5        43.0        46.1        49.5        50.7        53.4 
T.  143.8      166.1      197.2      214.0      230.5      229.8      213.6 

54.7 
207.5 

fe)   Solid  phase— NH4Br. 
%NH4Br.  57.1 

T  above  360°. 

As  has  been  found  with  the  other  alkali  metals,  the  curve  rises 
steeply  beyond  the  equimolecular  compound,  depositing  in  this  case 
pure  NH4Br.  It  was  impossible  to  obtain  a  tube  with  sufficiently 
little  NH4Br  so  that  two  layers  were  not  present.  To  find  the  limit 
of  the  two  layer  region  in  this  part  of  the  curve,  it  would  be  neces- 
sary to  employ  the  Beckman  freezing  point  method,  work  which  is 
beyond  the  scope  of  this  investigation. 

In  this  system  two  of  the  compounds — the  3AlBr3,  NH4Br  and 
the  equimolecular — are  stable  at  their  maxima  and  have  melting 
points  97°. 8  and  232°,  respectively.  The  compound  2AlBr,,  NH4Br 
gets  just  beyond  its  maximum  before  transition  to  the  equimolecular 
compound  occurs.  The  melting  point  for  this  compound  is  104°. 2. 

SYSTEM  AgBr-AlBr3 

Silver  bromide  was  prepared  by  precipitating  a  solution  of  c.p. 
HBr  with  silver  nitrate  in  a  dark  room.  The  precipitate  was  washed 
thoroughly  by  decantation  and  transferred  to  a  filter  paper,  where 
it  was  rewashed  till  free  from  bromide  ion.  The  product  was  care- 
fully dried  at  120°  and  then  heated  to  200°  to  decompose  any  com- 
pounds between  AgBr  and  HBr  which  might  be  present.  The  salt 
was  preserved  in  a  bottle  well  protected  from  the  light  and  tubes  for 
this  system  were  made  up  only  on  days  when  it  was  dry  but  not  too 
light. 

1  The  anomalous  behavior  of  lithium  will  be  taken  up  under  the  discussion 
of  results. 

20 


In  this  system  Isbekow  and  Plotnikow1  report  a  two  liquid  layer 
region  similar  to  the  ones  with  the  alkali  metals.  These  writers  give 
180°  as  the  critical  temperature  of  mixing,  but  mention  no  com- 
pounds. No  other  work  on  this  system  could  be  found. 

The  data  are  given  below  and  in  Fig.  2,  A. 

(a)  Solid  phase— AlBr3. 
%AgBr.  0.00        1.1          1.3 

T.  97.1        94.2       93.4 

(b)  Solid  phase— (?),  probably  2AlBrs,AgBr. 
%AgBr.  1.5          1.6 

T.  95.3        98.2 

(c)  Two  liquid  layers,  solid  phase  disappears  at  105°. 9. 

%AgBr.  3.0         4.5          5.0          7.6        13.4        14.7        16.2 

T.ofCoales.    120.4      156.3      161.9      183.6      173.0      159.7      139.7 

(d)  Solid  phase— 2AlBr3,AgBr. 

%AgBr.  18.7        20.7        22.1        25.6        26.4        27.8 

T.  108.6      111.3      112.8      115.6      116.9      117.8 

(e)  Solid  phase— AlBr3,AgBr. 

%AgBr.            30.2        32.0  35.6        37.3        39.2        40.9        42.7        46.8 

T.               125.2      135.0  154.6      164.1      174.1      182.3      189.5      206.8 

%AgBr.            50.2        50.6  52.0 

T.               215.8      214.2  210.3 

(f)  Solid  phase— AgBr. 
%AgBr.  54.9 

T.  319.0 

The  curve  is  rising  rapidly  beyond  the  50%  composition  with 
the  separation  of  the  neutral  salt,  as  was  the  case  with  the  alkali 
metals.  There  is  no  evidence  in  this  system  of  an  unstable  compound 
in  the  two  layer  region  as  evidenced  by  the  non-existence  of  a  second 
eutectic2  at  the  higher  composition  end  of  the  two  layer  region. 

The  results  here  recorded  show  the  existence  of  two  new  com- 
pounds—2  AlBr3,  AgBr  and  AlBr3AgBr.  The  first  is  unstable  at  its 
maximum,  undergoing  transition  at  11 8°. 4  to  the  equimolecular  com- 
pound. The  second  compound  has  a  melting  point  of  215°. 6  and 
is  stable  well  beyond  its  maximum.  The  critical  temperature  of  mix- 
ing was  found  to  be  186°,  agreeing  well  with  the  value  of  180°  found 
by  Isbekow. 

SYSTEM  MgBr2AlBr3 

Anhydrous  MgBr2  was  prepared  from  a  c.p.  sample  of  the  hexa- 
hydrate  by  the  method  proposed  by  Liebig.3  MgBr2  6H2O  forms  a 
stable  equimolecular  compound  with  NH4Br,  which  upon  gentle  heat- 
ing first  loses  all  of  the  water  of  hydration,  leaving  the  compound 
MgBr2,  NH4Br.  By  raising  the  temperature  this  compound  is  de- 
composed ;  and  after  strong  heating  only  MgBr3  is  left.  The  product 
so  obtained  gave  only  a  very  slight  basic  reaction,  indicating  prac- 
tically no  decomposition  to  oxide. 

1  Isbekow  and  Plotnikow,  loc.  cit. 

2  See  Roozeboom,  loc.  cit. 

3  Liebig,  Fogg.  19  137,  (1830). 

21 


No  previous  work  on  this  system  could  be  found  in  the  literature. 
Data  are  given  below. 

(a)  Solid  phase— AlBr3. 
%MgBr,.  0.00        0.6          1.4 

T.  97.1        96.8        96.5 

(b)  Solid  phase— 2AlBrs,MgBr2-(  ?). 

%MgBr2.  0.6          1.4         3.8         8.1        12.9        18.4        21.6        23.5 

T.  134.9      160.7      190.2      199.6      210.5      221.6      227.9      231.5 

Tubes  with  more  than  24%  MgBr2  could  not  be  brought  into 
solution  by  prolonged  heating  at  360°.  The  solid  phase  separated  in 
fine  crystals,  probably  the  neutral  salt  by  comparison  with  the  action 
of  the  monovalent  metal  systems  already  given.  It  will  be  noted  that 
the  curve  rises  rapidly  with  only  small  %'s  of  MgBr2,  a  fact  noted 
in  many  of  the  systems  which  follow. 

One  new  compound  was  found ;  probably  2AlBr3,  MgBr2,  since 
this  is  the  one  found  most  frequently  in  the  systems  with  the  divalent 
bromides  which  follow.  This  is  the  first  case  encountered  where 
transition  to  the  next  phase  occurs  at  such  a  composition  as  to  make 
the  true  nature  of  the  compound  uncertain.  To  remove  this  uncer- 
tainty it  would  be  necessary  to  analyze  the  solid  phase  as  has  been 
done  in  previous  work.1,2 

The  transition  of  the  compound  occurs  at  233°. 

SYSTEM  CaBr2-AlBr3 

The  salt  used  was  prepared  from  a  U.S. P.  sample  by  recrystal- 
lizing  once  from  distilled  water  and  carefully  decomposing  the  hydrate 
(hexa).  The  salt  after  drying  in  an  oven  at  120°  for  several  hours 
was  placed  in  a  Pyrex  tube  and  fused  to  insure  the  complete  removal 
of  water.  The  product  so  obtained  showed  only  a  slight  basic  reaction 
and  was  used  without  further  purification.  Extra  precautions  were 
again3  observed  in  the  preserving  of  the  salt  and  the  preparation  of 
tubes  for  this  system,  due  to  the  very  deliquescent  nature  of  anhy- 
drous CaBr2. 

No  previous  work  on  this  system  is  reported. 

The  data  are  given  below  and  in  Fig.  2. 

(a)  Solid  phase— AlBr3. 
%CaBr,.  0.00 

7.  97.1 

(b)  Solid  phase — xA!Br3,yCaBr2. 
%CaBr,  0.74 

T.  204.2 


1  Davidson,  Columbia  University  Dissertation,  (1920).    Also  Adler,  loc.  cit. 

2  Due  to  the  very  hygroscopic  nature  of  the  two  salts   involved,   it  was 
thought  best  not  to  attempt  analysis  on  the  solid  phase,  but  to  be  satisfied  with 
the  information  given  by  the  freezing  point  curve.    This  was  the  policy  adopted 
throughout   this    work. 

3  See  note  under  lithium  system  above. 

22 


Ft  G.  1ST 


A  (CBr4-AIBra)  Add  30°  to  Temp. 
B  (SnBr4-AI  Br3)  Subtract  50*  from  Temp. 
C  (AsBr3-AI  Br^      "       I50°   "       " 
D(SbBr3-AIBr3)      -      250°  »      » 
E  (BiBrrAlBr3)      «      350°  "      " 


80  100 

4c/ded  Bromide 


(c)  Two  liquid  layers;  solid  phase  disappears  at  208°. 8. 

No  temperatures  of  mixing  could  be  determined  for  this  system  as  even 
the  low  percentages  coalesced  at  temperatures  above  300°. 

(d)  Solid  phase— xA!Brs,yCaBr2. 
%CaBr2.  15.4 

T.  195.3 

(e)  Solid  phase— 2AlBrs,CaBr2. 

%CaBr2.  16.05      18.9        21.4        24.3        27.3        30.95      33.8 

T.  213.1      229.5      242.6      260.1      276.8      298.4      304.9 

(f  )   Solid  phase— CaBr2. 
%CaBr2.  33.8 

T.  398.0 

It  was  impossible  to  bring  more  than  34%  of  CaBr2  into  solu- 
tion even  at  450°.  The  curve  is  rising  rapidly  even  at  this  compo- 
sition, as  was  the  case  with  the  previous  systems.  The  solid  phase 
is  undoubtedly  CaBr2. 

There  is  an  unstable  compound  in  the  two  layer  region  as  evi- 
denced by  the  two  eutectic  points,  one  on  either  side  of  the  two 
layer  region.  The  compound  2AlBr3,  CaBr2  is  just  stable  at  its 
maximum  and  has  a  melting  point  of  306°. 


SYSTEM  BaBr2-AlBr3 

The  salt  used  was  a  c.p.  sample  recrystallized  once  from  distilled 
water  and  dried  at  120°. 

Isbekow  and  Plotnikow1  report  that  this  salt  is  insoluble  in 
aluminium  bromide.  The  present  work  shows  that  this  is  not  the 
case.  These  writers  probably  mistook  for  pure  BaBr2  the  solid  phase 
— 2AlBr3,BaBr2 — which  separates  even  in  the  lower  percentages. 
Since  the  curve  rises  steeply  to  the  two  layer  region  where  the  solid 
phase  disappears  at  269°. 4,  this  error  in  interpretation  can  be 
readily  explained. 

The  data  for  this  system  are  given  below. 

(a)  Solid  phase— AlBr3. 
%BaBr2.  0.00 

T.  97.1 

(b)  Two    liquid    layers;    solid    phase    disappears    at    269°. 4. 

As  with  CaBr2,  no  temperatures  of  coalescence  could  be  determined. 

(c)  Solid  phase— 2AlBr3,BaBr2. 

%BaBr2.  .88      12.0        15.2        18.3        21.2        24.2        28.03 

T.  269.4      269.4      269.4      276.7      292.0      310.0      335.0 

The  lower  limit  of  the  two  layer  region  could  not  be  determined 
by  means  of  closed  tubes,  so  was  not  attempted.  One  new  compound, 
very  likely  2AlBr3,  BaBr2,  similar  to  the  one  with  CaBr2,  has  been 
found.  On  account  of  the  high  temperature,  no  percentages  higher 
than  twenty-eight  were  made  up. 

1  Isbekow  and  Plotnikow,  loc.  cit. 

24 


SYSTEM  ZnBr2-AlBr3 

The  salt  used  was  prepared  from  a  c.p.  sample  by  one  recrystalli- 
zation  from  water.  The  product  after  drying  at  120°  was  transferred 
to  a  bent  Pyrex  tube  and  distilled.  The  anhydrous  salt  was  quickly 
placed  in  a  glass  stoppered  bottle  and  kept  in  an  oven  during  use. 

Isbekow  reports  that  ZnBr2  gives  a  completely  homogeneous 
solution  with  aluminium  bromide.  A  great  tendency  towards  super- 
cooling was  found  in  this  system;  and  hence,  although  66%  could 
be  readily  brought  into  solution,  a  glass  was  formed  upon  cooling 
which  would  not  crystallize  even  after  months  of  standing.  This 
same  phenomenon  made  the  taking  of  freezing  points  in  this  system 
especially  difficult  even  with  the  lower  percentages. 

Data  for  this  system  are  given  immediately  below. 

(a)  Solid  phase — AlBrs. 

%ZnBr3.  0.00        2.6          5.6        11.3        14.5 

T.  97.1        95.5        94.4        87.6        83.5 

(b)  Solid  phase — 2AlBr3,  ZnBr2. 

%ZnBr2.  11.3        12.1        14.5        20.6        26.7        30.0        35.1 

T.  95.1        96.1        99.3      104.1      108.8      110.4      110.6 

One  new  compound — 2AlBr3,  ZnBr2 — with  a  melting  point  of 
111°. 5  has  been  found.  On  account  of  the  formation  of  the  glasses 
previously  noted,  the  work  was  not  extended  beyond  35%. 


SYSTEM  CdBr2-AlBr3 

The  salt  used  was  a  c.p.  sample  recrystallized  once  from  dis- 
tilled water  and  carefully  dried  at  120°. 

Isbekow  reports  that  this  system  gives  no  two  layers,  but  gives 
no  further  data. 

Data  are  given  below  and  in  Fig.  3,  B. 

(a)  Solid  phase — AlBr3. 
%CdBr2.  0.00        0.7 

T.  97.1        95.6 

(b)  Solid  phase— 2AlBr3,CdBr2. 

%CdBr2.             J.I          1.8  4.5  5.2         9.0        11.6        13.04      19.3 

T.               140.7      168.1  189.6  190.0      192.9      195.4      197.0      204.2 

%CdBr2.           26.0       28.9  33.1  35.3 

T.               217.4      221.2  224.0  223.1 

(c)  Solid  phase — CdBr2. 
%CdBr2.  35.3 

T.  234.9 

One  new  compound,  2AlBr3,  CdBr2,  with  melting  point  224° 
was  found. 

This  system  is  especially  interesting  from  the  standpoint  of  the 
phase  rule  on  account  of  the  marked  tendency  towards  the  formation 
of  two  liquid  layers  in  the  same  percentage  region  where  two  layers 
were  noted  in  the  systems  above.  In  this  system,  however,  the  two 

25 


layers  exist  only  in  the  metastable  region,1,2  and  merely  introduces  a 
pushing  up  of  this  portion  of  the  curve  leading  to  extreme  flatness. 

SYSTEM  HgBr-AlBr3 

Mercurous  bromide  was  prepared  by  a  method  exactly  similar  to 
the  one  used  to  prepare  AgBr.  This  salt  was  also  preserved  in  a 
dark  place  and  tubes  made  up  and  worked  with  only  in  subdued  light. 

Isbekow  reports  that  this  salt  is  soluble  in  aluminium  bromide 
to  the  extent  of  3-4%  at  220°.  This  has  been  confirmed  and  the 
system  extended  beyond  70%. 

Data  are  given  below  and  in  Fig.  3,  D. 

(a)  Solid  phase— AlBr3. 
%HgBr.  0.00        0.6 

T.  97.1        96.6 

(b)  Solid  phase— (?),  probably  AlBrs,  HgBr. 
%HgBr.  1.2          1.7 

T.  161.5      225.4 

(c)  Two  liquid  layers;  solid  phase  disappearing  at  238°. 1. 

%HgBr.  3.3    (No  other  points  could  be  determined  on  account  of  the 

T.  275.5    }  high  temperature  and  consequent  danger  of  explosion. 

(d)  Solid  phase— AlBr3,  HgBr. 

%HgBr.  33.9        35.0        40.5        44.4        48.6        53.2        54.7        59.6 

T.  242.1      243.9      250.1      255.1      259.7      256.6      252.9      241.4 

(e)  Solid  phase— HgBr. 
%HgBr.  62.7        66.1 

T.  243.7      281.3 

One  new  compound,  AlBr3,  HgBr,  similar  to  the  one  obtained 
with  silver,  though  less  stable,  melting  at  261°,  has  been  found. 

The  curve  is  rising  so  rapidly  at  66%  that  further  work  was  not 
attempted.  There  seems  little  probability  that  further  compounds 
exist  since  the  behavior  of  the  higher  percentages  is  strictly  analogous 
to  that  of  silver  and  lithium. 

SYSTEM  HgBr2-AlBr3 

Mercuric  bromide  was  prepared  by  the  direct  action  of  bromine 
on  mercury  which  was  kept  under  water  to  control  the  action.  The 
salt  was  recrystallized  twice  from  distilled  water  and  carefully  dried. 
The  product  was  found  to  melt  sharply  at  241°. 5. 

Isbekow  reports  homogeneous  solutions  for  this  system  which 
have  been  confirmed;  also  one  new  compound — 2AlBr3,  HgBr2, 
which  exists  in  two  crystalline  modifications — has  been  found. 

Data  for  this  system  are  given  below  and  in  Fig.  3,  A. 

(a)  Solid  phase — AlBr8. 
%HgBr2.  0.00        1.44 

T.  97.1        95.8 


1  See  Miller,  loc.  cit. 

2Hildebrand,  J.A.C.S.,  38  1452   (1916)   discusses  the  factors  which  affect 
immiscibility. 

26 


(b)  Solid  phase— 2AlBr,,  HgBra. 

%HgBr2.             3.8  4.6  7.1  9.8  10.1        13.7        17.9        20.1 

T.   (Stable)      94.3  ....  95.9  ....  96.7       98.7  100.1 

T.  (Unstable)    ....  93.5  94.7  95.8        99.1      100.1 

%HgBr2.           25.8  28.7  31.4  32.2  32.85      37.7        40.3 

T.  (Stable)     102.8  103.6  104.1  ....  103.9      103.1  101.9 

T.  (Unstable)  101.9  102.5  ....  102.6        100.7 

(c)  Solid  phase— HgBr2. 

%HgBr,.  44.2  45.2  49.4        59.6        62.8        75.7        83.1        88.0 

T.  118.8  123.2  145.5      175.0      183.6      206.8      217.1      224.4 

%HgBr2.  94.0  98.1  100.0 

T.  232.9  239.1  241.5 

The  compound — 2AlBr3,  HgBr2 — existing  in  two  crystalline 
forms  is  stable  well  beyond  its  maximum,  but  is  not  a  very  stable 
compound,  as  evidenced  by  the  flatness  of  the  curve  around  33%. 
The  melting  points  of  the  two  modifications  are:  stable,  103°. 9; 
unstable,  102°. 8.  In  the  tubes  of  higher  composition  there  was  con- 
siderable darkening  which  made  the  taking  of  melting  points  more 
difficult,  but  the  reproducibility  was  well  within  the  limits  estab- 
lished above.1  In  the  diagram  (Fig.  3,  A.)  only  the  points  for  the 
stable  form  of  the  compound  are  plotted. 


SYSTEM  TlBr-AlBr3 

Thallous  bromide  was  prepared  by  precipitating  a  solution  of 
c.p.  HBr  with  pure  thallous  sulfate.  After  thorough  washing  by 
decantation,  the  precipitate  was  transferred  to  a  filter  paper  and 
again  washed  to  remove  all  sulfate.  The  salt  was  carefully  dried  at 
120°  and  then  heated  to  200°  to  insure  the  decomposition  of  any 
compounds  between  TIBr  and  HBr.  The  salt  so  prepared  is  light 
yellow  in  color,  resembling  freshly  precipitated  AgBr.  The  salt, 
although  not  ultra  pure,  was  deemed  of  sufficient  purity  for  this  in- 
vestigation. 

No  previous  work  on  this  system  could  be  found.  The  data 
are  given  below  and  in  Fig.  1,  A. 

(a)  Solid  phase— AlBr8. 
%TlBr.  0.00 

T.  97.1 

(b)  Two  liquid  layers;  solid  phase  disappears  at  103°.9. 
%TlBr.  0.6        21.4        22.4 

T.ofCoales.     260°        260°       118.4 

(c)  Solid  phase— xAlBr,,  tyTlBr. 
%TlBr.  24.4 

T.  99.9 

(d)  Solid  phase— 2AlBr3,  TIBr. 

%TlBr.  24.4       26.03      26.9        28.4       30.7        32.7 

T.  104.4      105.9      106.7      108.1      110.6      111.8 


1  See  "Precision  of  Measurements." 

27 


(e)  Solid  phase— AlBr,,  TIBr. 

%TlBr.  35.4        37.6        40.0        40.7        42.4        46.3        48.2        51.0 

T.  126.8      142.1      157.7      160.4      171.8      192.9      203.4      207.9 

%TlBr.  52.1        53.3 

T.  200.8      193.1 

(f)  Solid  phase— TIBr. 
%TlBr.  55.0        55.2 

T.  213.8      215.9 

The  curve  is  rising  rapidly  at  55%  with  the  deposition  of  TIBr. 

The  resemblance  of  this  system  to  those  of  the  alkali  metals 
and  silver  is  striking,  a  point  to  be  discussed  more  fully  after  the 
presentation  of  results. 

Three  new  compounds:  xAlBr3,yT!Br ;  2AlBr3,TlBr;  and 
AlBr3,TlBr,  have  been  isolated.  The  first  of  undetermined  composi- 
tion breaks  up  into  two  liquid  layers;  the  second  is  stable  at  its 
maximum,  but  undergoes  transition  to  the  equimolecular  compound 
just  beyond  this  composition.  The  equimolecular  compound  is  very 
stable,  having  a  melting  point  of  210°. 

The  lower  limit  of  the  two-layer  region  could  not  be  estab- 
lished by  the  method  here  employed. 

SYSTEM  CBr4-AlBr3 

The  carbon  tetrabromide  used1  in  this  system  was  recrystallized 
once  from  absolute  alcohol  and  dried  in  a  vacuum  desiccator.  The 
product  melted  sharply  at  90°.  1,  which  agrees  sufficiently  well  with 
the  values  recorded  in  the  literature2  to  permit  its  use  in  this  investi- 
gation. 

This  work  confirms3  the  existence  of  two  modifications  of 
carbon  tetrabromide,  with  a  transition  point  of  48°. 4.4  Isbekow 
reports  complete  miscibility  for  this  system. 

No  other  work  on  this  system  could  be  found.  Data  are  given 
below  and  in  Fig.  4,  A. 

(a)  Solid  phase— oA!Br8. 

%CBr.  0.00        1.02       2.7        11.5        23.03 

T.  97.1        96.6       94.5        85.7       72.5 

(b)  Solid  phase— &AlBr3. 

%CBr4.  29.5        42.5        47.6        49.5        55.9 

T.  68.5        56.7        52.3        50.2       44.0 

(c)  Solid  phase— frCBn. 
%CBr4.  66.1        67.6 

T.  44.7        46.1 

(d)  Solid  phase— aCBn. 

%CBr4.  73.7       85.5       90.3      100.0 

T.  51.7       68.8       77.6       90.1 


1The  author  wishes  to  express  his  thanks  to  Prof.  M.  T.  Bogert  for  the 
loan  of  the  sample. 

2Abegg,  Hand.  Anorg.  Chem.  3  (2)  113.  Bolas  and  Graves,  J.C.S.,  23 
161  (1870). 

3  Schwarz,  Roozeboom,  Heterogene  Gleichgewicht,  i  127 ;  Rothmund,  Zeit. 
Phys.  Chem.,  24  714  (1897). 

4  The  value  recorded  above  is  slightly  higher  than  previous  ones  and  could 
not  be  checked  because  of  the  small  quantity  of  CBr4  available. 

28 


No  compounds  were  isolated,  which  was  to  be  expected  from 
the  non-polar  nature  of  the  two  salts.  The  existence  of  the  two 
forms  of  aluminium  bromide  has  been  confirmed  in  this  system. 

SYSTEM  SnBr4-AlBr3 

Stannic  bromide  was  prepared  by  the  direct  action  of  bromine  on 
powdered  tin.  The  salt  was  distilled  twice  from  excess  tin  and 
preserved  in  a  tightly  stoppered  flask.  The  salt  crystallizes  to  a 
beautiful  white  solid,  melting  sharply  at  3 10.1 

Isbekow  reports  complete  miscibility  for  this  system,  which  has 
been  confirmed  in  this  work. 

Data  are  given  below  and  in  Fig.  4,  B. 

(a)  Solid  phase — 0AlBr3. 

%SnBr4.  0.00        1.9          2.5        13.4        22.7 

T.  97.1        96.0        94.3        85.4        76.4 

(b)  Solid  phase— &AlBr3. 

%SnBr  .  35.4        49.2        56.7        71.2        74.4        76.0 

T.  65.7        53.7        45.8        29.7        25.5        22.3 

(c)  Solid  phase — SnBr4. 
%SnBr,.  82.5        93.3       100.0 

T.  23.4        27.6        31.0 

The  two  forms  of  aluminium  bromide  noted  above  are  shown 
to  be  present  in  this  system.  The  failure  to  form  compounds  is,  as 
would  be  expected,  from  the  nature  of  the  two  components. 


SYSTEM  SnBr2-AlBr3 

The  stannous  bromide  used  was  prepared  by  the  action  of  c.p. 
hydrobromic  acid  on  30-mesh  tin,  after  the  reduction  of  stannic 
bromide  by  excess  tin  had  failed.  After  the  tin  was  completely  dis- 
solved, the  solution,  which  was  a  bright  yellow  color,  was  evaporated 
to  small  volume  and  allowed  to  crystallize.  The  crystals,  which  were 
a  hydrate  of  a  complex  compound,  xHBr,  ySnBr2,  were  freed  from 
mother  liquor  on  a  porous  plate,2  transferred  to  a  wide  Pyrex  tube, 
sealed  at  one  end,  and  gently  heated  to  drive  off  water  and  HBr. 
The  temperature  was  carried  well  above  300°  to  insure  the  complete 
removal  of  HBr.  The  salt,  which  was  pale  yellow,  was  transferred 
to  a  glass  stoppered  bottle  and  kept  in  an  oven  at  120°  during  use. 

The  product  obtained  by  the  above  method  contained  a  small 
quantity  of  oxide  formed  by  the  decomposition  of  the  complex 


1  Various  values  between  25°. 5  and  33°    are  recorded   in  the   literature. 
Thus,  Bertholet,  Thermochimie,  2  156   (1897)  ;   Carnelley  and  O'Shea,  Chem. 
News,  36  264  (1877)  ;  Rayman  and  Preis,  Lieb.  Ann.,  223  323  (1884). 

2  Attempts  were  made  to  crystallize  from  acetone  and  alcohol,  but  resulted 
in  the  destruction  of  both  salt  and  solvent. 

29 


hydrate.  Several  samples  from  two  different  preparations  melted 
sharply  at  232°.  The  best  previous  value  which  could  be  found  was 
215°.5.1 

No  previous  work  on  this  system  could  be  found  recorded. 
Data  are  given  bolew  and  in  Fig.  2,  C. 

(a)  Solid  phase— AlBr,. 

%SnBr,.  0.00        0.35        0.81        1.07 

T  97.1        96.9        96.8        96.3 

(b)  Solid  phase— (?),  probabl«y  2AlBrs,  SnBr2. 
%SnBr,.  0.8          1.07        1.45 

T.  121.6  137.1      152.8 

(c)  Two  liquid  layers;  solid  phase  disappears  at  161°. 1. 
%SnBr,.  2.1          3.4         4.7        10.4        10.7        13.34 
T.ofCoales.    169.1      187.3      198.0      202.4      201.4      185.4 

(d)  Solid  phase— 2AlBr3,  SnBr2. 

%SnBr2.  16.4        16.6        18.5        24.7        28.3        30.7        34.95 

T.    '  162.2      162.7      164.3      178.5      190.2      197.8      202.0 

%SnBr,.  41.5        43.7 

T.  181.8      175.4 

(e)  Solid  phase— AlBrs,  SnBr2. 

%SnBr2.  44.8  48.3  52.4  55.2  59.6  62.8 

T.  175.0  179.8  179.1  172.9  164.4  158.3 

(f  )  Solid  phase— SnBr2. 

%SnBr2.  71.2  78.1  82.9  90.8  96.25  100.0 

T.  '  175.1  195.9  206.4  220.9  228.2  232.0 

The  critical  temperature  of  mixing  for  the  two-layer  region  is 
204°.5. 

Two  new  compounds,  2AlBr3,SnBr2  and  AlBr3,SnBr.,,  have 
been  isolated  with  melting  points  205°  and  183°  respectively.  The 
compound  2AlBr3,SnBr2  is  the  most  stable  of  the  two,  as  evidenced 
by  the  sharpness  of  its  maximum  in  comparison  to  that  of  the 
equimolecular  one. 

There  is  no  evidence  of  an  unstable  compound  in  the  two-layer 
region,  this  system  resembling  the  one  with  silver. 

SYSTEM  PbBr2-AlBr3 

Lead  bromide  was  prepared  by  a  method  similar  to  the  one 
used  for  HgBr  above.  The  salt  was  recrystallized  once  from  dis- 
tilled water  and  carefully  dried  at  120°. 

No  previous  work  on  this  system  could  be  found  in  the  literature. 
Data  are  given  below  and  in  Fig.  2,  B. 

(a)  Solid  phase— AlBr,. 
%PbBr2.  0.00 

T.  97.1 

(b)  Solid  phase  (?),  probably  2AlBr3,  PbBr2. 
%PbBr2.  0.63 

T.  191.9 

(c)  Two  liquid  layers;  solid  phase  disappears  at  210°.4. 

'Abegg,  Hand.  Anorg.  Chem.,  3  (2)  571. 

30 


No  temperatures  of  coalescence  could  be  determined,  even  those 
for  low  percentages  lying  above  the  boiling  point  of  AlBr3. 

(d)  Solid  phase— 2AlBr3,  PbBr2. 

%PbBr2.  16.95      20.1        23.7        27.0        30.5        32.5        37.0        39. 

T.  211.9      220.4      235.5      253.5      267.7      272.5      266.9      257. 

%PbBr,.  43.6        45.5 

T..  241.6      234.9 

(e)  Solid  phase— PbBr2. 
%PbBr2.  52.3        57.7 

T.    '  268.4      296.8 

The  curve  is  rising  rapidly  at  58%  with  the  deposition  of  PbBr2 
as  solid  phase.  There  is  no  evidence  of  an  unstable  compound  in 
the  two-layer  region;  this  system  resembling  that  of  barium  above. 

One  new  compound — 2AlBr3,PbBr2 — has  been  isolated,  which 
is  stable  at  its  melting  point,  274°. 

SYSTEM  PBr3,AlBr3 

The  phosphorous  tribromide  used  was  an  E.  &  A.  sample,  twice 
redistilled.  The  middle  fraction,  boiling  constantly  at  175°,  was 
collected  and  preserved  in  sealed  tubes,  which  were  opened  only 
long  enough  to  fill  one  bulb  at  a  time. 

Due  to  the  very  hygroscopic  nature  of  this  compound,  the  curve 
could  not  be  carried  beyond  50%.  Beyond  this  composition  fine 
crystals,  probably  POBr3,  were  deposited,  which  invalidated  the  work 
on  higher  points.  In  the  region  investigated,  no  compounds  were  iso- 
lated. No  previous  work  on  this  system  could  be  found.  Data  are 
given  below. 

(a)  Solid  phase — aA!Br8. 

%PbBrs.          0.00          7.35        14.4        22.4 
T.  97.1        91.3        84.1        76.6 

(b)  Solid  phase— 6AlBr3. 

%PBr,.  31.5        44.0        48.2        49.1 

T.  67.3        53.6        47.0        45.4 

The  two  modifications  of  aluminium  bromide  are  again  apparent 
in  this  system. 

SYSTEM  AsBr3-AlBr3 

The  arsenic  tribromide  used  in  this  work  was  prepared  from 
a  "pure"  E.  &  A.  sample  by  two  distillations.  The  middle  fraction 
from  the  second  distillation  was  taken  and  preserved  in  a  sealed  tube. 
The  salt  melted  sharply  at  32°. 8,  and  was  used  in  the  fused  condition 
throughout. 

Isbekow  reports  complete  miscibility  for  these  two  components. 

Data  for  the  complete  system  are  given  below  and  in  Fig.  4,  C. 

(a)  Solid  phase — aA!Br3. 
%AsBr3.  0.00        5.3        19.3 

T.  97.1       94.5       81.1 

(b)  Solid  phase— Z?AlBr3. 

%AsBr,.  32.8        40.1        44.8        46.4        56.9        68.05 

T.  69.5        65.7        61.9        60.9        52.8        41.7 

31 


(c)   Solid  phase — AsBr3. 
%AsBr3.  80.9        96.1       100.0 

T.  28.2        32.2        32.8 

No  compound  was  isolated  in  this  system,  a  result  to  be  ex- 
pected from  the  nature  of  the  two  components.  The  two  modifica- 
tions of  AlBr3  are  again  shown  here. 

SYSTEM  SbBr,-AlBrQ 


Antimony  tribromide  was  prepared  by  a  method  analogous  to 
the  one  used  in  the  preparation  of  aluminium  bromide  cited  above. 
The  product  was  distilled  twice  and  used  without  further  purification. 
Samples  melted  sharply  at  96°. 6.1 

Isbekow  reports  complete  miscibility  for  these  two  salts.  This 
has  been  confirmed  in  this  work,  as  well  as  finding  one  new  com- 
pound, AlBr3,  SbBr3,  M.P.,  85°. 2. 

Data  are  given  below  and  in  Fig.  4,  D. 

(a)  Solid  phase— AlBr3. 

%SbBr3.  0.00        8.2        15.2        19.2        28.6 

T.  97.1        94.7        92.3        90.3        82.9 

(b)  Solid  phase— AlBr3,  SbBr3. 

%SbBr3.  37.1        46.5        49.3        53.5        63.5        66.4 

T.  80.5        84.2       85.1        84.3        78.8        76.9 

(c)  Solid  phase— SbBr3. 

%SbBr3.    '        72.5        75.7        81.6        86.7        92.8      100.0 
T.    '  72.9        76.3        81.9        86.5        91.3        96.6 

That  a  compound  should  be  found  in  this  system  is  not  surpris- 
ing after  consideration  of  the  conductivity  results  given  by  Isbekow 
and  Plotnikow2  for  this  system.  They  find  much  greater  conduc- 
tivity for  antimony  tribromide  in  aluminium  bromide  than  for  arsenic 
tribromide  in  the  same  solvent.  The  values  for  antimony  tribromide 
rise  to  a  maximum  in  the  vicinity  of  fifty  molecular  per  cent,  as 
would  be  expected  to  be  the  case3  if  a  compound  were  present,  as  has 
been  found  in  this  work. 

SYSTEM  BiBr3-AlBr3 

Bismuth  tribromide  was  prepared  by  the  direct  action  of  bromine 
on  metallic  bismuth.  Due  to  the  slowness  of  the  reaction  an  appara- 
tus to  allow  bromine  to  be  continually  in  contact  with  the  metal  was 
employed.4  The  product,  which  is  bright  yellow  in  color,  was  dried 
from  the  last  traces  of  bromine,  placed  in  a  Pyrex  tube  bent  at  an 
angle  of  45°,  and  slowly  distilled  into  the  open  half  of  the  tube. 

1  Various    values    are    given    in    the    literature.      Thus:    90° -94°,    Abegg, 
Hand.  Anorg.  Chem.,  3   (3)   590;  90°,  Cooke,  J.  B.,    (1877),  p.  284;  94°  .0, 
Serullas,  Ann.  Chim.  Phys.,  38  322  (1828). 

2  Isbekow  and  Plotnikow,  loc.  cit.,  p.  335. 

8  See   Gross,    Columbia   University   Dissertation    (1919)    for    a   discussion 
of  the  effect  of  compound  formation  on  ionization  and  hence  conductivity. 
4  An  apparatus  with  reflux  condenser  attached  was  used. 

32 


After  cooling,  the  product  was  recovered  by  gently  tapping  the  tube. 
The  salt  so  obtained  melted  sharply  at  220°. 4.1 

Isbekow  finds  that  BiBr3  is  completely  miscible  in  all  propor- 
tions with  aluminium  bromide.  Data  for  the  complete  system  are 
given  below  and  in  Fig.  4,  E. 

(a)  Solid  phase— AlBra. 
%BiBr3.  0.00        4.4        10.6 

T.  97.1        96.8        95.5 

(b)  Solid  phase— AlBr8,  BiBrs. 

%EiEra.  10.6        23.6        32.5        43.5        46.0        51.1        57.7 

T.  98.5      119.2      135.5      150.2      152.0      153.1      147.8 

%BiBr3.  65.6 

T.  137.3 

(c)  Solid  phase— BiBrs. 

%BiBr3.  71.5        79.7        88.0      100.0 

T.  156.9      180.1      202.7      220.4 

One  new  compound,  AlBr3,BiBr3,  has  been  isolated  with  a  melt- 
ing point  of  153°. 

The  compound  with  bismuth  is  much  more  stable  than  the 
corresponding  one  with  antimony,  showing  that  there  is  an  increase 
in  stability  of  compounds  with  increasing  metallicity  of  the  added 
component.  This  point  will  be  taken  up  under  the  discussion  of  re- 
sults. 

SYSTEM  CrBr3-AlBr3 

Chromic  bromide  was  obtained  in  black,  shining  crystals  by 
the  direct  action  of  bromine  on  heated  chromium  metal.  Only  a 
limited  quantity  could  be  made  by  this  method;  but  due  to  the 
almost  complete  insolubility  of  this  salt  in  aluminium  bromide,  this 
was  sufficient  to  determine  the  limit  of  solubility. 

Less  than  0.5%  of  chromic  bromide  could  be  brought  into 
solution  at  200°.  The  result  agrees  with  the  results  obtained  by 
Miller2  for  the  corresponding  chloride  system. 

SYSTEM  MnBr2-AlBr3 

Anhydrous  manganous  bromide  was  obtained  from  a  "pure" 
E.  &  A.  sample  by  recrystallizing  once  from  distilled  water  and  care- 
fully decomposing  the  hydrate.  The  salt  was  then  fused,  broken  up, 
and  preserved  in  a  glass  stoppered  bottle  in  an  oven  at  120°  during 
use. 

Isbekow  reports  no  two  layers  for  this  system.  In  this  work 
the  curve  was  carried  to  30%,  at  which  composition  the  curve  was 
rising  steeply  with  the  deposition  of  MnBr2. 

Data  are  given  below  and  in  Fig.  3,  C. 
(a)   Solid  phase — AlBr3. 
%MnBr2.  0.00        0.68 

T.  97.1        96.3 


1  The  best  previous  value  recorded  is  215°. 

2  Miller,   loc.   cit. 

33 


(b)   Solid  phase— 2AlBrs,  MnBr2. 

%MnBr2.             0.68        1.98       4.6  5.65        9.4        13.8        17.5        20.6 

T.               127.1      171.6      199.1  199.8      204.6      210.8      217.6      223.8 

%MnBr,.           24.0        25.8        28.0  29.6 

T.               232.9      237.7      241.7  242.6 

Tubes  with  28%  or  more  MnBr2  separated  fine  crystals  which 
could  not  be  brought  into  solution  below  300°.  This  behavior  is 
similar  to  that  noted  in  the  magnesium  and  cadmium  systems  above. 
The  flat  portion  of  the  curve  from  3-9%  recalls  the  case  of  cadmium 
especially.  This  system,  as  will  be  noted,  shows  the  tendency  to- 
wards the  formation  of  two  layers  never  realized  in  the  stable  region. 

One  new  compound,  probably  2AlBr3,MnBr2,  by  analogy  to 
cadmium,  has  been  found.  This  compound  undergoes  transition  near 
28%  to  the  neutral  salt. 

SYSTEM  NiBr2-AlBr3 

Anhydrous  nickel  bromide  was  made  by  treating  pure  nickel 
lumps  with  c.p.  HBr.  The  solution  so  obtained  was  filtered, 
evaporated  to  small  volume,  and  allowed  to  crystallize.  The  crystals 
of  the  hydrate  were  carefully  decomposed  at  120°,  and  the  partially 
dry  salt  heated  to  250°  to  insure  the  decomposition  of  any  com- 
pounds between  HBr  and  NiBr2.  The  salt  was  a  finely  divided, 
deep-brown  powder,  resembling  Fe2O3. 

Isbekow  reports  the  complete  insolubility  of  NiBr2  in  aluminium 
bromide.  In  this  work  it  was  found  possible  to  bring  0.54%  into 
solution  at  300°,  but  0.78%  could  not  be  made  to  dissolve  after 
several  hours'  heating  at  360°. 

SYSTEM  FeBr2-AlBr3 

Ferrous  bromide  was  prepared  from  a  "pure"  sample  by  heat- 
ing in  a  Pyrex  tube,  slowly,  at  first,  to  decompose  the  hydrates,  and 
finally  by  raising  the  temperature  to  500°.  The  product,  although 
of  rather  doubtful  purity,  was  deemed  of  sufficiently  good  quality 
to  test  the  solubility  for  this  system.  Tubes  with  ferrous  bromide 
were  deep  red  in  color,  which  made  the  taking  of  exact  melting 
points  exceedingly  uncertain.  As  far  as  could  be  determined  from 
the  relative  melting  points  on  a  few  tubes,  the  curve  resembled  that 
given  by  magnesium,  and  more  especially  cadmium.  The  curve  is 
rising  rapidly  at  first  with  the  deposition  of  fine  crystals  followed 
by  a  flat  portion  indicating  a  tendency  toward  the  formation  of  two 
layers.  This  type  of  curve  might  be  expected  from  the  position 
of  ferrous  iron  in  the  "electrode  potential"  series,  since  it  is  only 
slightly  below  cadmium. 

The  limit  of  solubility  lies  near  20%,  for  20.8%  could  not  be 
brought  into  solution  after  heating  at  300°  for  an  hour. 

No  previous  work  on  this  system  could  be  found  recorded  in 
the  literature. 

34 


DISCUSSION  OF  RESULTS 
(a)  Compounds  Isolated. 

The  double  salts  isolated  in  this  investigation  are  given  below 
in  Table  I.  Only  one  compound — AlBr3,KBr — has  been  previously 
reported.  The  elements  in  the  table  are  arranged  in  the  order  as 
found  in  the  periodic  system. 

TABLE  I 

Li— 7AlBr3,LiBr ;  2AlBr3,LiBr;  AlBr3,LiBr. 

Na— xA!Brs,yNaBr ;  7AlBr3,2NaBr;  2AlBr3,NaBr;  AlBr3,NaBr. 

K— xA!Br3,yKBr;  2AlBr3,KBr;  AlBr3,KBr. 

NH4— xA!Br3,yNH4Br;  3AlBr3,NH4Br;  2AlBr3,NH4Br ; 

AlBr3,NH4Br. 

Ag— 2AlBr3,AgBr;  AlBr3,AgBr. 
Ca— xA!Br3,yCaBr2 ;  2AlBr3,CaBr2. 
Ba— 2AlBr3,BaBr2. 
Mg— 2AlBr3,MgBr2. 
Zn— 2AlBr3,ZnBr2. 
Cd— 2AlBr3,CdBr2. 
Hg'— AlBr3,HgBr. 
Hg"— 2AlBr3,HgBr2. 

Tl'— xAlBra,yTlBr;  2AlBr3,TlBr;  AlBrs,TlBr. 
C — None. 
Sn"  "—None. 

Sn"— 2AlBr3,SnBr2 ;  AlBr3,SnBr2. 
Pb— 2AlBr3,PbBr2. 
P"  '—None. 
As"  '—None. 
Sb— AlBr3,SbBr3. 
Bi— AlBr3,BiBr3. 
Cr"  '—None. 
Mn"— 2AlBr3,MnBr2. 
Ni — None. 
Fe"— 2AlBr3,FeBr2,  (?). 

In  agreement  with  the  main  point  of  the  theory  (e.g.,  'diversity* 
factor),  the  number  and  complexity  of  the  compounds  isolated  de- 
creases to  a  minimum  with  elements  near  aluminium  in  the  electrode 
potential  series  and  increases  again  below  this  metal.  Thus  we  find 
the  alkali  metal  bromides  giving  the  greatest  number  and  the  most 
complex  compounds  with  aluminium  bromide.  With  magnesium, 

35 


compound  formation  has  fallen  off  considerably,  for  we  find  only  one 
compound  and  that  with  a  very  narrow  range  of  stability.  With  tin 
and  lead  well  below  aluminium  we  find  compound  formation  has  in- 
creased in  extent  and  continues  to  do  so  until  we  reach  a  maximum 
again  in  the  silver  system.  It  becomes  apparent  from  a  considera- 
tion of  the  above  table  that  in  fused  salt  mixtures  just  as  in  the  sys- 
tems previously  investigated  that  the  diversity  of  the  positive  radicals 
is  the  main  factor  influencing  the  formation  of  addition  compounds ; 
although  compound  formation  in  salt  mixtures  is  much  more  influ- 
enced by  subsidiary  factors  (e.g.,  atomic  volume,  unsaturation, 
valence)  than  in  any  systems  previously  studied.  Thus  we  find  that 
solubility  in  the  present  systems  reaches  a  minimum  with  nickel  and 
chromic  bromides,  salts  of  metals  well  below  aluminium  in  the  elec- 
trode potential  series ;  while  zinc  and  magnesium  bromides  are  much 
more  soluble  in  aluminium  bromide  than  would  be  the  case  if  the 
diversity  factor  were  the  only  one  exerting  an  effect.  The  small 
atomic  volume  of  these  latter  elements  suggests  that  this  factor  is 
exerting  a  counterbalancing  effect.  This  point  will  be  discussed 
under  a  separate  heading  below. 

Throughout  this  investigation,  the  type  of  compound  isolated  is 
strikingly  dependent  upon  the  valence  of  the  metal  in  the  added 
bromide.  Thus  we  find  with  the  univalent  metals  (e.g.,  silver,  potas- 
sium, thallium)  that  the  equimolecular  compound  is  by  far  the  most 
stable ;  although,  as  was  to  be  expected,  other  compounds  are  present. 
With  the  divalent  metals  (e.g.,  calcium,  cadmium,  magnesium),  the 
predominant  type  of  compound  is  2AlBr3,MBr2;  while  with  the 
trivalent  elements  such  as  antimony  and  bismuth,  we  find  a  return  to 
the  equimolecular  type.  In  the  case  of  the  quadrivalent  elements 
(e.g.,  tin  and  carbon),  we  find  no  apparent  tendency  towards  com- 
pound formation.1 

Here  again,  as  in  previous  work,2  the  temperature  of  fusion  of 
the  added  component  has  a  marked  effect  on  solubility.  The  higher 
lies  the  fusion  point  of  the  added  salt,  the  less  is  the  solubility. 
Thus  nickel  bromide  is  far  less  soluble  in  aluminium  bromide  than 
is  ferrous  bromide;  although,  from  their  respective  positions  in  the 
electrode  potential  series,  we  should  expect  relatively  the  same  solu- 
bility. Further  illustrations  of  this  behavior  are  obtained  by  a 
comparison  of  the  solubilities  in  aluminium  bromide  of  barium  with 
calcium  bromide,  lead  with  stannous  bromide,  and  mercurous  with 
mercuric  bromide. 

Unsaturation,  which  is  also  a  subsidiary  factor  in  compound  for- 
mation distinct  from  the  factor  of  valence  mentioned  above,  has  a 
marked  influence  in  the  case  of  thallous  bromide.  Thus  this  com- 
pound would  be  expected  to  have  a  relatively  low  solubility  in  alumi- 

1  This  does  not  necessarily  mean  that  unstable  complexes  do  not  exist  in 
the  solution.  See  Kendall,  Booge,  and  Andrews,  J.A.C.S.,  39  2303  (1917). 

2  Davidson,  loc.  cit. 

36 


nium  bromide  from  the  position  of  the  metal  in  the  electrode  poten- 
tial series.  However,  we  find  this  salt  has  a  solubility  of  the  same 
order  as  that  of  the  alkali  metals,  which  must  be  accounted  for  by 
the  unsaturated  character  of  the  thallium  atom  in  thallous  com- 
pounds. This  same  factor  likewise  leads  to  the  formation  of  a 
larger  number  of  compounds  in  the  case  of  thallium  above  and  also 
in  the  case  of  stannous  bromide.  (See  Fig.  2,  C.)  This  point  will 
be  found  more  fully  discussed  by  Miller1  in  a  comparison  of  solubili- 
ties in  chloride  and  bromide  systems. 

One  further  point  needs  to  be  mentioned  before  passing  to  the 
discussion  of  specific  factors ;  namely,  the  effect  of  increasing  atomic 
weight  on  compound  formation.  As  has  been  pointed  out  by  Abegg 
and  Bodlander,2  the  tendency  towards  combination  increases  with 
increasing  atomic  weight.  This  is  strikingly  shown  by  a  comparison 
of  the  systems  of  the  fifth  group  of  the  periodic  system  (i.e.,  the 
phosphorous  family)  with  aluminium  bromide.  Thus,  disregarding 
phosphorous  whose  action  could  not  be  completely  determined,3  and 
comparing  the  last  three  members  (i.e.,  arsenic,  antimony,  and  bis- 
muth), we  find  that  compound  formation  regularly  increases.  There 
is  no  indication  of  a  compound  with  arsenic,  only  a  very  unstable 
equimolecular  one  with  antimony,  but  a  very  stable  equimolecular  one 
with  bismuth.  This  same  effect  is  apparent  in  the  systems  of  lead 
and  stannous  tin.  (See  Fig.  2,  Band  C.)  Thus  we  find  the  2AlBr3, 
MBr2  compound  in  these  systems  is  much  the  stabler  in  the  lead 
system  as  well  as  having  the  widest  range  of  existence, 
(b)  Immiscibility  and  Internal  Pressure. 

Low  solubility  accompanied  by  the  formation  of  two  liquid  layers 
has  been  observed  for  similar  portions  of  the  curve  in  several  systems 
above.  This  phenomenon,  although  frequently  observed  in  systems 
of  organic  liquids  with  water,4  has  never  been  so  clearly  noted  for 
fused  salt  mixtures.  In  order  to  make  clear  why  such  a  phenomenon 
should  be  observed  in  certain  systems  and  not  in  others,  it  is  necessary 
to  inquire  into  the  rules  which  may  be  said  to  govern  immiscibility 
in  general. 

Hildebrand  and  his  co-workers5  have  shown  that  in  any  complete 
study  of  solubility  certain  factors  of  a  purely  physical  nature  must 
be  taken  into  account  along  with  the  chemical  factors  (e.g.,  com- 
pound formation)  ordinarily  accepted  as  governing  solubility  rela- 


1  Miller,  loc.  cit. 

2  Abegg  and  Bodlander,  Zeit.  Anorg.  Chem.,  20  453  (1899)  ;  39  330  (1904). 
8  It  seems  highly  improbable  that  any  compounds  exist  in  the  region  which 

was  not  investigated  when  we  consider  the  slightly  polar  nature  of  the  two 
compounds. 

4  The  most  familiar  case  of  this  kind  is  found  in  the  system  phenol-water. 
For  other  cases  see  Roozeboom,  Heterogene  Gleichgewicht,  2   (2)    168. 

5  Hildebrand,  J.A.C.S.,  38  1470  (1916)  ;  30  2297  (1917)  ;  41  1067  (1919)  ; 
42  2180,  2213  (1920). 

37 


tions.  The  most  important  of  these  purely  physical  factors  are 
internal  pressure  and  'polar'1  nature,  which  will  be  discussed  in  light 
of  the  present  investigation. 

By  internal  pressure  is  understood  the  force  which  with  the 
external  pressure  opposes  the  thermal  pressure  arising  from  the 
kinetic  energy  of  the  molecules.  Expressed  in  terms  of  Van  der 
Waal's  equation,  it  is  the  a/v2  term.  As  has  been  pointed  out  by 
Hildebrand,  this  quantity,  although  a  fundamental  one,  is  not  experi- 
mentally determinable,  but  can  be  obtained  by  indirect  means  only. 
Various  methods  for  calculating  this  quantity  have  been  proposed  by 
several  investigators,2  leading  to  rather  diverse  values.  From  a 
consideration  of  these  correlated  values,  Hildebrand  was  able  to  lay 
down  several  general  rules  regarding  solubility  which  may  be  sum- 
marized as  follows:  (1)  The  solubility  of  a  non-polar  substance 
in  a  non-polar  liquid  of  equal  internal  pressure  is  that  calculated 
on  the  basis  of  Raoult's  law.  (2)  When  the  internal  pressures  are 
unequal,  the  solubility  is  less,  depending  upon  the  difference  of  inter- 
nal pressure.  (3)  When  one  substance  is  polar  and  the  other  non- 
polar,  the  solubility  is  less  than  that  indicated  by  Raoult's  law.  (4) 
When  both  substances  are  polar  (leading  frequently  to  the  forma- 
tion of  isolable  compounds),  the  solubility  is  usually  greater  than 
that  indicated  by  Raoult's  law. 

In  general,  fused  salts  are  highly  polar,  leading  often  to  high 
association  factors  and  frequently  to  excellent  conductivity  in  the 
fused  condition.  Their  internal  pressures  are  high  as  a  rule,  resulting 
in  low  solubility  in  organic  solvents,  but  leading  to  high  solubility  in 
water  and  other  fused  salts.  From  time  to  time  cases  have  arisen 
where  pure  salts  in  the  fused  state  have  been  found  to  be  poor  con- 
ductors and  have  low  dielectric  constants,  a  manifestation  of  slightly 
polar  nature  and  low  internal  pressure.  Thus  aluminium  bromide3  in 
the  fused  state  is  practically  a  non-conductor,  as  are  also  stannic 
bromide  and  mercuric  chloride.  As  a  rule  salts  of  those  elements  near 
the  center  of  the  periodic  system  are  poorer  conductors  in  the  fused 
state  than  those  of  more  diverse  elements.4 

In  Table  2  below  are  given  the  values  for  internal  pressures  of 
the  bromides  used  in  this  investigation  in  so  far  as  data  are  avail- 
able for  their  calculation.  The  values,  though  merely  relative  (being 
in  error  as  much  as  a  thousand  atmospheres  in  some  cases),  are 
nevertheless  accurate  enough  to  make  comparisons  valid. 


*For  a  discussion  of  'polar'  nature,  see  Bray  and  Branch,  J.A.C.S.,  55 
1440  (1913),  and  Lewis,  ibid.,  35  1448  (1913). 

'Walden,  Zeit.  Phys.  Chem.,  66  385  (1909)  ;  Traube,  ibid.,  68  291  (1909)  ; 
Mathews,  J.  Phys.  Chem.,  17  603  (1913)  ;  Lewis,  Phil.  Mag.,  28  (6)  104  (1914). 

'Isbekow  and  Plotnikow,  loc.  cit. 

*Of  all  salts  the  alkali  halides,  exactly  those  whose  radicals  are  most 
diverse,  are  the  best  conductors  in  the  fused  state. 

38 


TABLE  2 

Substance  Internal  pressure                      Method 

SnBr,  1770  atmos.                    Hildebrand1 

PBr3  2230  " 

CBr4  2300  " 

AlBr3  2550  " 

AsBr3  2820  " 

SbBr3  3240  " 

HgBr2  4400  "                         Van  Laar2 

BiBr3  4860  "                         Hildebrand 

SnBr2  8990  " 

ZnBr2  9550  " 

HgBr  10000  "                         Van  Laar 

CdBr2  10700  "                         Hildebrand 

KBr  21700  " 

AgBr  25000  " 

NaBr  28400  " 

LiBr  35600  " 

3CaBr2  No  data 

From  this  table  it  becomes  at  once  apparent  that  there  is  as 
wide  a  range  of  internal  pressure  for  fused  salts  as  has  been  found 
for  organic  substances,4  With  salts  of  nearly  the  same  internal  pres- 
sure (e.g.,  stannic  tin,  carbon,  arsenic)  aluminium  bromide  gives 
completely  miscible  solutions,  as  is  to  be  expected.  When  the  internal 
pressure  of  the  added  salt,  however,  has  reached  a  value  of  nearly 
10,000  atmospheres,  we  find  the  type  of  curve  given  by  cadmium5 
(See  Fig.  3,  B)  where  there  is  a  marked  tendency  toward  the  forma- 
tion of  two  liquid  layers  never  realized6  in  the  stable  region. 

In  some  cases  in  point  of  fact  (e.g.,  mercurous  and  stannous), 
even  when  the  internal  pressure  of  the  added  salt  is  10,000  atmo- 
spheres, we  find  two  distinct  layers.  With  salts  whose  internal  pres- 
sures are  greater  than  ten  thousand  atmospheres,  we  obtain  a  two 
layer  region  which  may  or  may  not  contain  an  unstable  compound7 


1  Wherever  possible  the  equation  given  by  Hildebrand,  J.A.C.S.,  41  1067 
(1919  was  used.     The   equation  is:   p  =  20.65   (5200 -f  30£b)/V*,   where   "pi" 
is  the  internal  pressure,  tb    the  boiling  point  of  the  compound,  and  ¥20  the 
molecular  volume  at  20°. 

2  Van  Laar,  J.  de  Chim.  Physik.,  14  3  (1916)  gives  the  method  for  finding 
the  value  of  a  for  some  compounds  which  makes  it  possible  to  determine  the 
internal  pressure  by  the  a/v2  relation. 

8  Judging  from  the  high  temperature  of  fusion  of  this  and  the  following 
compounds :  TIBr,  PbBr2,  BaBr5,  and  FeBra,  their  internal  pressures  undoubt- 
edly lie  above  12,000  atmospheres. 

*  See  table  of  relative  internal  pressures  given  by  Hildebrand,  loc.  cit. 

B  The  similar  behavior  of  manganese  and  magnesium  makes  it  seem  likely 
that  the  internal  pressures  of  these  bromides  are  nearly  the  same  as  that  of 
cadmium. 

8  See  Miller,  loc.  cit.  for  an  interesting  case  of  this  kind. 

7  See  notes  under  the  silver  and  sodium  systems. 

39 


The  rule  of  Hildebrand  that  the  greater  the  difference  in  internal 
pressure  the  greater  is  the  immiscibility  is  completely  justified  in  the 
case  of  the  fused  salt  mixtures  studied  above.  Certain  minor  dis- 
crepancies are  to  be  noted  in  some  few  systems,  a  result  to  be  ex- 
pected from  the  present  uncertain  position  of  fused  salts  in  Hilde- 
brand's  theory.  The  action  of  zinc  bromide  in  not  giving  two  layers, 
although  its  internal  pressure  is  similar  to  that  of  stannous  bromide, 
must  be  referred  to  the  atomic  volume  factor  discussed  immediately 
below. 

Before  passing  to  the  next  topic,  it  might  be  well  to  call  atten- 
tion to  the  fact  that  in  all  cases  where  two  layers  are  noted  this  region 
occurs  with  the  higher  percentages  of  aluminium  bromide.  This 
is  best  explained  by  pointing  out  that  it  is  only  in  this  region  that 
we  are  truly  comparing  the  internal  pressure  of  aluminium  bromide 
with  that  of  the  added  bromide ;  since  with  higher  percentages  we  are 
forced  to  compare  the  internal  pressure  of  a  compound  such  as 
xA!Br3,yMBr  with  that  of  another  compound  or  else  to  that  of 
the  pure  added  component.  The  effect  of  adding  a  substance  of 
low  internal  pressure  to  one  of  high  internal  pressure,  especially  if 
a  compound  is  formed,  is  to  make  the  internal  pressure  of  the  com- 
pound higher  than  that  of  the  lowest  pressure  constituent. 

(c)   Solubility  and  Atomic  Volume. 

Another  subsidiary  factor  which  must  be  dealt  with  in  light  of 
this  investigation  is  the  effect  of  the  atomic  volumes  of  the  metallic 
constituents  upon  solubility  data  as  well  as  ionization  in  so  far  as 
the  latter  can  be  predicted  from  this  work.  This  factor  has  been 
the  subject  of  numerous  investigations1  in  the  past  few  years.  Of 
these  brief  mention,  in  passing,  needs  to  be  made  of  the  work  of 
Holmes.  This  writer  has  attempted  to  show  that  there  is  a  close 
relation  existing  between  the  ratio  of  the  diameters  of  the  molecules 
and  immiscibility.  He  presents  numerous  cases  to  show  the  validity 
of  his  claim,  but  since  he  has  completely  neglected  the  motion  of  the 
molecules  due  to  their  kinetic  energy,  his  work  is  of  little  value  for 
comparisons  here. 

W.  L.  Bragg2  has  recently  advanced  his  "Law  of  Atomic  Diame- 
ters" in  which  he  postulates  spherical  atoms  with  fixed  diameters  for 
all  elements  as  determined  by  crystal  structure  measurements.  He 
advances  the  idea  that  the  diameter  of  a  molecule  of  a  compound  is 
the  sum  of  the  diameters  of  its  respective  atoms.  Pease3  takes  excep- 
tion to  this,  pointing  out  the  necessity  for  considering  the  con- 
figuration of  the  respective  atoms  as  well.  The  value  of  the  atomic 
diameter  factor  apart  from  the  atomic  volume  factor  in  solubility 
relations  is  taken  up  by  Miller4  in  his  discussion  of  solubility  in 

i  Holmes,  J.C.S.,  103  2147   (1913),  also  J.C.S.,  113  263   (1918);  Harkins 
and  Hall,  J.A.C.S.,  38  194  (1916)  ;  Richards,  J.A.C.S.   43  1584  (1921). 
2W.   L.   Bragg,   Phil.   Mag.,    (6)    40   169    (1920). 
'Pease,  J.A.C.S.,  44  769  (1922). 
4  Miller,  loc.  cit. 

40 


chloride  and  bromide  systems.  All  that  needs  to  be  mentioned  here 
in  this  connection  is  that  where  the  difference  in  the  atomic  diameters 
of  the  positive  radicals  is  large,  the  tendency  towards  immiscibility  is 
far  greater.  Thus  the  alkali  metals  (e.g.,  sodium,  potassium)  whose 
atomic  diameters  are  great  in  comparison  with  that  of  aluminium, 
form  two  layer  regions  as  pointed  out  above;  while  zinc  with  an 
atomic  diameter  of  the  same  magnitude  as  aluminium  does  not  give 
two  layers  in  the  system  studied  above. 

The  main  point  to  be  discussed  here  is  the  relation  between 
atomic  volume  and  compound  formation,  and  its  bearing  upon  ioni- 
zation  in  so  far  as  the  latter  can  be  judged  from  the  present  work. 
The  anomalous  behavior  of  lithium  bromide  in  not  giving  two  liquid 
layers  with  aluminium  bromide  must  be  referred  to  this  factor.  Cer- 
tain rules  only  briefly  presented  before  will  be  first  taken  up  in 
greater  detail. 

If  we  compare  elements  of  the  same  type  (i.e.,  with  the  same 
number  of  electrons  in  the  outer  shell),  then  we  should  expect  those 
with  smaller  atomic  volumes,  owing  to  the  greater  attractive  forces 
at  the  surface,  to  give  addition  compounds  with  neutral  molecules 
more  readily  than  those  with  larger  atomic  volumes,  where  the  attrac- 
tive forces  at  the  surface  are  relatively  weak.  This  expectation  is 
in  accordance  with  the  actual  facts.  Thus  lithium  has  the  smallest 
atomic  volume  of  any  of  the  alkali  metals ;  and  lithium  ion  is  more 
highly  hydrated  in  aqueous  solution  than  the  sodium  or  potassium  ion. 
In  the  same  way  magnesium  salts  are  more  highly  hydrated  in  solu- 
tion than  the  corresponding  compounds  of  calcium,  barium,  or  stron- 
tium. Those  metals  which  exhibit  exceptionally  small  atomic  volumes 
(e.g.,  aluminium,  cobalt  and  platinum)  are  exactly  those  which  give 
salts  conspicuous  in  the  formation  of  molecular  complexes  (e.g.,  the 
alums,  the  cobaltamines  and  the  platinocyanides). 

In  the  case  of  lithium  bromide,  which  does  not  form  two  liquid 
layers  with  aluminium  bromide  as  would  be  expected  from  the  great 
difference  in  the  internal  pressures  of  these  two  salts,  we  must  look 
to  the  atomic  volume  factor  for  an  explanation  of  this  behavior.  We 
are  dealing  here  with  two  metals  both  with  small  atomic  volumes,  and 
hence  the  tendency  towards  the  formation  of  molecular  complexes  is 
unusually  high ;  in  fact,  so  great  that  the  electrical  forces  at  the  surface 
of  the  molecules  are  sufficient  to  overcome  the  difference  in  internal 
pressure  tending  to  cause  immiscibility.  Although,  as  has  been  pointed 
out  previously,  the  complexities  arising  in  the  case  of  fused  salts 
make  an  explanation  of  their  behavior  uncertain,  yet  it  seems  evident 
in  the  case  just  cited  that  we  can  ascribe  the  action  to  one  single 
factor,  i.e.,  atomic  volume. 

The  abnormally  high  solubility  of  zinc  and  magnesium  bromides 

41 


in  aluminium  bromide  must  be  ascribed  to  this  same  cause;  since,  if 
the  diversity  factor  were  the  only  criterion  of  their  action,  we  should 
expect  minimum  solubility  of  these  salts  in  aluminium  bromide. 

As  a  final  illustration  in  this  connection,  the  exceptional  behavior 
of  fluorine  in  the  series  of  the  halogens  may  be  considered.  Although 
fluorine  is  the  most  electronegative  member  of  this  series,  hydrogen 
fluoride  exhibits  less  ionic  instability  (in  other  words  is  less  "polar") 
than  the  remaining  hydrogen  halides.  This  abnormality  may  logic- 
ally be  ascribed  to  the  small  atomic  volume  of  fluorine.1  On  the 
other  hand,  this  small  atomic  volume  also  induces  the  formation  of 
molecular  complexes  (e.g.,  hydrofluosilicic  acid,  potassium  hydrogen 
fluoride)  to  a  much  greater  extent  than  is  the  case  with  the  other 
hydrogen  halides. 

The  next  consideration  is  the  effect  of  atomic  volume  on  the 
ionic  instability  of  the  complexes.  Whatever  view  we  hold  as  to 
the  constitution  of  the  atom,  the  fact  remains  certain  that  the  con- 
straint on  an  electron  at  the  surface  of  a  charged  atom  increases 
with  decreasing  atomic  volume.2  Hence  with  positive  atoms  of  large 
volume  (e.g.,  the  alkali  metals)  separation  of  electric  charges  should 
take  place  much  more  readily  than  with  atoms  of  small  atomic  volume 
such  as  carbon.  As  a  matter  of  fact  we  find  that  the  compounds  of 
the  alkali  metals  followed  by  those  ot  the  alkaline  earths  show  the 
most  pronounced  tendency  towards  ionization  of  all  simple  com- 
pounds, while  carbon  gives  the  least  "polar," 

We 'have  here  an  important  factor  which  must  be  considered  in 
any  discussion  of  ionization.  Where  the  atomic  volume  is  large, 
there  is  a  maximum  tendency  towards  ionization,  but  a  minimum 
tendency  towards  the  formation  of  molecular  complexes.  On  the 
other  hand,  where  the  ionic  volume  is  small,  the  original  tendency 
towards  ionization  is  less,  but  a  greater  tendency  is  evidenced  towards 
the  formation  of  molecular  complexes ;  the  "ionic  instability"  of  which 
is  regularly  more  pronounced  than  that  of  the  simple  salt,  as  has 
already  been  shown.3 

Ultimately,  of  course,  the  "diversity"  factor  (i.e.,  the  difference 
in  positive  and  negative  nature  of  the  constituent  radicals)  dealt  with 
fully  in  previous  work,  must  be  referred  to  this  basic  question  of 
ionic  volume,  since  the  "electrode  potential"  of  any  element  against  a 
standard  concentration  of  its  ions  will  be  fundamentally  dependent 
upon  the  "structure"  of  the  atom ;  this  structure  determining  the  mag- 

iSee  Langmuir,   J.A.C.S.,  41  907    (1919). 

2  Close  up  to  a  charged  sphere,  the  electric  force  is  related  to  the  radius 
of  the  sphere  and  will  be  greater  at  the  surface  of  a  small  sphere  than  at  that 
of  a  large  one. 

'Kendall  and  Gross,  J.A.C.S.,  43  1416  (1921). 

42 


nitude  of  the  forces  existent  between  the  nuclear  charge  and  the 
exterior  electrons,  thus  making  the  electrode  potential  a  function  of 
the  atomic  volume.  Our  knowledge  of  the  inner  fabric  of  the  atom  is 
far  too  fragmentary  at  present  to  allow  us  to  establish  any  definite 
quantitative  connection  between  the  "diversity"  and  atomic  volume 
factors.  However,  it  is  interesting  and  important  to  recognize  the 
fact  that,  in  general,  the  elements  with  very  large  atomic  volumes  are 
grouped  at  the  positive  and  negative  extremities  of  the  electromotive 
series,  while  those  with  very  small  atomic  volumes  are  collected  around 
the  center. 


SUMMARY 

In  the  present  investigation  freezing  point  curves  for  twenty- 
five  different  bromides  in  aluminium  bromide  as  a  solvent  have  been 
determined.  From  an  examination  of  the  curve  so  obtained,  it  is 
evident  that  as  in  previous  work  of  this  kind  the  "diversity"  factor 
(i.e.,  the  difference  in  character  of  the  positive  radicals  as  measured 
by  their  position  in  the  "electrode  potential"  series)  is  again  the 
governing  factor  in  the  formation  of  addition  compounds  between 
fused  salt  pairs.  Thus  the  most  extensive  compound  formation  was 
found  with  the  alkali  metals,  falling  off  as  the  positive  radical  ap- 
proached aluminium,  and  increasing  to  a  second  maximum  with  silver. 

In  this  work  on  fused  salts  it  has  been  shown  that  it  is  neces- 
sary to  take  into  account  more  fully  than  in  previous  work  the  effect 
of  certain  subsidiary  factors  such  as  internal  pressure,  atomic  volume, 
unsaturation,  valence  and  temperature  of  fusion  of  the  pure  compo- 
nents which  often  exercise  a  counterbalancing  on  the  ordinary  course 
of  compound  formation  and  solubility.  The  effect  of  internal  pres- 
sure and  atomic  volume  have  been  fully  discussed. 

During  the  course  of  the  investigation  thirty-two  new  com- 
pounds have  been  isolated  and  a  new  modification  of  aluminium 
bromide  has  been  discovered. 


43 


VITA 

Eugene  D.  Crittenden  was  born  at  Saline,  Michigan,  on  May  1, 
1898,  where  he  attended  the  grammar  and  high  schools.  From  1915- 
1919  he  attended  Michigan  State  Normal  College,  receiving  his  A.B. 
degree  in  December,  1919.  During  the  last  three  years  there  he  was 
assistant  in  Chemistry. 

In  1918  he  was  sent  to  the  Officers'  Training  Camp  at  Fort 
Sheridan,  111.,  receiving  the  rank  of  2nd  Lt.  Inf.  in  September  of 
that  year. 

In  1920  he  entered  the  graduate  school  of  Columbia,  holding 
a  lecture  assistantship  in  Chemistry.  In  June,  1920,  he  received  his 
Master's  degree  and  is  at  present  an  instructor  in  General  Chemistry 
at  Columbia. 

He  is  a  member  of  Phi  Lambda  Upsilon  and  Sigma  Xi. 


45 


Makers 
Syracuse,  N.  Y. 

PAT.  W  21,  1908 


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